There are a number of things that these two mountains could signify, with each suggestion not being necessarily exclusive of the others. Tidiman (1996: 143-44) succinctly summarizes scholarly opinion by grouping these as: (1) the colour of the sun rising or setting; (2) a gateway to the mythological abode of the gods; (3) the two pillars Boaz and Jachin from the First Temple; (4) a theocratic alliance of some nature; and, (5) two geographical mountains around Jerusalem. To make the best decision, we need context.
Literary context places this verse as initiating the seventh and final vision of Zechariah's heptadic vision cycle (Kline, 1996: 10). It follows on from Vision 6 at 5:1-11, the vision of the flying megillah and the flying ephah, which composite vision depicts the purging of lawbreakers from the land of Yehud at 5:1-4, and their removal to the land of Shinar at 5:5-11. The context of this composite vision at 5:1-11 is both fascinating and relevant, and requires a brief overview.
Part One of Vision 6 starts from within the precincts of the First Temple. Three spaces are suggested via the 20 x 10 cubit dimensions given for the flying megillah at 5:2. These are:
(a) the 'debir', where the two large cherubim guarding the Ark fit the 20 x 10 cubit dimensions (1 Kings 6:23-27);
(b) the 'ulam' or porch, whose dimensions were 20 x 10 cubits and where the two pillars Boaz and Jachin stood (1 Kings 6:3); and,
(c) the inner courtyard, where the altar of burnt offering stood with dimensions of 20 x 10 cubits (2 Chronicles 4:1).
All three spaces have merit and are possibly intended, but with primacy clearly given to (a) due to the fact that the megillah (a cipher for the Decalogue covenant; see 5:3) is 'flying', and the two large cherubim are winged guardians. Meyers and Meyers entertain this view as a possibility while exploring others (1987: 280-81). How does the megillah 'fly'? Because it is a symbol for the two tables of the Mosaic covenant that rested within the Ark, it flies through the two small winged cherubim atop the 'kapporet' of the Ark, and through the two large winged cherubim guarding the Ark in the 'debir'.
Part Two of Vision 6 sees this flying megillah at 5:1-2, which the Meyers suggest is a word-picture for the Ark of the Covenant, morph into a flying ephah at 5:5-6,9. Stead (2009: 197) aptly summarizes the flying ephah as an anti-ark, borne by anti-cherubim, taken to an anti-temple in an anti-Jerusalem. This view was anticipated by Barker (1978: 24). The picture is the antithesis of the earlier flying megillah, which is the Ark opened and the Covenant enacted (Kline, 1994: 5).
Vision 6 is one of movement. The movement can be read as passing from the space of the 'debir', out to the porch, past the altar of burnt offering, and finally out of the land itself. The movement is outward from the Temple.
It is easy to see how this outward movement equates to rituals performed on the Day of Atonement. This is true for the sequence of spaces covered, and for the underlying ritual purpose, which is the dual process of purgation (5:1-4) and elimination (5:5-11). This dual process is vital to understanding that Vision 6 covers the entire text of chapter 5 (so Rogland, 2014: 93-107). It is a mistake to split the text of chapter 5 into two separate visions because it fails to appreciate the dual aspects of the single ritual behind it. See Milgrom (1991: 1044-45) for further reading on purgation and elimination rites.
This overview of Vision 6 then sits within the literary framework of the heptad. The structure of the seven visions is chiastic (Kline, 1991: 179-192). We therefore need to look at the symbolism in the first vision in order to unravel that encountered in the last.
Vision 1 is set by the 'metsulah' at 1:8, a word of contested derivation of which by far the greater evidence falls in favour of it being 'the watery deep' (Boda, 2016: 124-25; Rogland, 2016: 72)). So we are looking at a sea of some description as part of an elemental backdrop to the first vision.
How does the idea of a sea at Vision 1 in any way help to clarify what our two mountains might be at Vision 7? Two suggestions will be considered here.
First, what is apparent up to this point in our reading model is that the visions take place in a Temple setting that serves as a liturgical model of the cosmos (Seybold, 1974: 65-75). The 'metsulah' of the first vision at 1:8, being 'the watery deep', matches the Sea from the First Temple (see 1 Kings 7:23-26). This Sea was, like the two mountains at Zech 6:1, made entirely from brass. This brazen Sea had an important ritual function. But it also was weighted with symbolic signification. One of its symbolic meanings relates, I suggest, to the land. Specifically, the brazen Sea is, among other things, a symbolic representation of the Mediterranean Sea, which is the western border of the land and the place where the sun sets. This forms a key boundary, geographically, of the land of the covenant people.
This reading of 'metsulah' as the brazen Sea would lend itself, then, to the two mountains of 6:1 being the two pillars outside the entrance to the First Temple (see Curtis, 2006: 143; Tiemeyer, 2015: 246). Both the two brazen pillars and the brazen Sea were encountered before entry to the Temple proper. They were constructed of brass whereas everything within the Temple proper was fashioned from gold. This differentiation in metals serves to delineate sacred space. The two mountains would then, as the 'metsulah', lend themselves to a geographical interpretation. This will be investigated shortly.
Returning to the 'metsulah', it is also symbolic of the cosmic deep of ancient near East mythology. This results in at least two symbolic readings: one geographical (the Mediterranean Sea) and one mythological (the cosmic deep).
If the 'metsulah' of 1:8 symbolizes (1) the Mediterranean Sea via the First Temple object of the brazen Sea, and (2) the cosmic deep, what would the two mountains at 6:1 - symbolic of the two brazen pillars Boaz and Jachin - represent geographically and mythologically?
Mythologically, they would represent the entrance to the abode of the gods from ancient Near East literature and iconography. This, then, is echoed in an Israelite context in the position of the two pillars Boaz and Jachin, which stood directly before the entrance to the First Temple itself, the House of Yahweh. Geographically, they might represent the interior of the land, which was mountainous. This last point is not very strong. Keil (1873: 576) believes the two mountains to be Mount Zion and the Mount of Olives, thus locating them in Jerusalem. For further background to the two mountains, see the excellent summary in Boda (2016: 361-67).
The second point concerning literary context is temporal rather than spatial, and is built around the heptadic vision cycle supposedly occurring in a single night, from dusk till dawn (see Wolters, 2014: 48-49). If Vision 1 occurs as the sun is going down, Vision 7 would be set as the sun is rising. This assumed literary setting would fit quite nicely with the symbolism suggested for the 'metsulah' at 1:8 and the two mountains at 6:1.
If the 'metsulah' - both cosmic deep and the Mediterranean Sea - were to lead us symbolically to the western border of the land, we might indeed imagine Vision 1 as set at sundown, as the sun sinks into the Mediterranean Sea. This would then lend the vision cycle a progressive movement eastward as we ascend from the coast to the Temple. At a closer level of reading, we might imagine the sun going down as the prophet, standing before the former Temple precincts, contemplates where the brazen Sea once stood. He would be looking westward from the brazen Sea towards the Temple just behind it.
The vision cycle would then reach its conclusion at and then beyond the Temple, with the sun rising between the mountains toward the east. This reading would favour a cosmic reading for the two mountains, though two specific mountains might also be intended. Keil's two mountains could come into play here. In the absence of a definite identification for these latter, however, we would have to go back to the material from which the two mountains at 6:1 are constructed in order to supplement a geographical reading.
The two brazen mountains, from a Temple perspective, would be the two pillars Boaz and Jachin. The Temple was oriented toward the east. Coming out from the Temple proper (the 'hekhal'), one would see the sun rise from between the two brass pillars. The sun would be rising opposite Mount Zion and over the Mount of Olives.
In answer to what the two mountains at 6:1 might be, several possibilities overlap.
(1) They could represent sunrise, and the colour of the sun rising through the material of which they were constructed. Burnished brass gives off a gleam akin to the rising sun. The vision cycle read as a progressive temporal heptad would support this view. If Vision 1 is set at sundown, Vision 7 would be set at sunrise.
(2) They almost certainly have a mythological connection through the ancient Near Eastern milieu that Israel has drawn from to construct its own cosmology. They would then be the entrance to the divine abode. The chiastic structure of the seven visions would support this reading, with the paired Visions 1 and 7 having a cosmic setting.
(3) The connection to the pillars Boaz and Jachin holds its own via their grounding in an Israelite expression of a Semitic cosmology, through the material from which they were constructed, through their orientation facing east and the rising sun, and through a symbolic reading that links them to the land itself.
(4) Finally, the possibility that they represent two specific mountains cannot be ruled out entirely. If the polyvalent reading of the Sea at Vision 1 allows an equation of the 'metsulah' at 1:8 with the Mediterranean Sea in the west, an equivalent reading of the two mountains at 6:1 as two topological features toward the east remains a possibility.
Of these, the one that embodies in itself all of the other possibilities suggested here is surely the best choice. Clearly, this specification would point squarely to option (3), the two pillars Boaz and Jachin. They face sunrise. They have both mythological and geographical signification. They are situated upon one mountain looking directly east at another in very close proximity. And, most importantly, they are constructed entirely of brass.
That four chariots emerge from between them is the next step, which will not be covered here. In conclusion, however, we must bear in mind that this reading is firmly grounded in a First Temple ideology and iconography. It should occasion little surprise that Zechariah should write from this position. From a priestly family, with a detailed knowledge of the collected cultic traditions and writings of his people, and writing at a time where temple reconstruction had just been initiated and undertaken by the community, we would expect him to use objects of significance from the First temple as motivating symbols to his audience.