Is John writing about the same event or two separate events?
Answer: These are two entirely separate events.
Revelation 6:15-16: "Then the kings of the earth and the great men and the commanders and the rich and the strong and every slave and free man hid themselves in the caves and among the rocks of the mountains; and they said to the mountains and to the rocks, 'Fall on us and hide us from the presence of Him who sits on the throne, and from the wrath of the Lamb'"
The imagery in these passages, that of hiding in the mountains and calling for rocks to fall on them, are symbols used for judgment on Jerusalem. We can know this because the language parallels events prophesied by Isaiah long before, referring to the imminent destruction of sinful Jerusalem. Notice the similarity:
Isaiah 2:19, 21: “Men will go into caves of the rocks And into holes of the ground Before the terror of the LORD And the splendor of His majesty, When He arises to make the earth tremble... In order to go into the caverns of the rocks and the clefts of the cliffs Before the terror of the LORD and the splendor of His majesty, When He arises to make the earth tremble."
Understandably, though the object of God's wrath is Judah and Jerusalem, the rest of the known world is terrified by the carnage wrought by the Romans during the siege. The prophet Hosea used similar words to describe the judgment against the Northern Kingdom of long ago:
Hosea 10:7-8: “Samaria will be cut off with her king Like a stick on the surface of the water. Also the high places of Aven, the sin of Israel, will be destroyed; Thorn and thistle will grow on their altars; Then they will say to the mountains, 'Cover us!' And to the hills, 'Fall on us!'”
Even more significant, let us not miss the fact that, during His Crucifixion, Christ had this to say to the people of Israel:
Luke 23:26-31: “[Jesus said,] “Daughters of Jerusalem, stop weeping for Me, but weep for yourselves and for your children. For behold, the days are coming when they will say, ‘Blessed are the barren, and the wombs that never bore, and the breasts that never nursed.’ Then they will begin TO SAY TO THE MOUNTAINS, ‘FALL ON US,’ AND TO THE HILLS, ‘COVER US.’ For if they do these things [crucify the Messiah] when the tree is green, what will happen when it is dry? [when God has left them]'” (emphasis added).
It, therefore, seems most plausible that the words spoken in Revelation 6:15-16 refer to the judgment against Jerusalem.
Now, let us contrast this with the symbolism of Revelation 19.
Revelation 19:15-19: "From His mouth proceeds a sharp sword with which to strike down the nations, and He will rule them with an iron scepter. He treads the winepress of the fury of the wrath of God the Almighty. 16And He has a name written on His robe and on His thigh: KING OF KINGS AND LORD OF LORDS.
17Then I saw an angel standing in the sun, and he cried out in a loud voice to all the birds flying overhead, “Come, gather together for the great supper of God, 18so that you may eat the flesh of kings and commanders and mighty men, of horses and riders, of everyone slave and free, small and great. 19Then I saw the beast and the kings of the earth with their armies assembled to wage war against the One seated on the horse, and against His army."
Verse 15 reveals messianic imagery. The sharp sword coming out of Christ's mouth, also seen in Revelation 1:16, is His Word — Scripture. The Bible is "sharper than any two-edged sword." He will strike every worldly nation, ruling "with a rod of iron." These figurative statements refer to (messianic) Psalm 2:
Psalm 2:9: You shall break them with a rod of iron, You shall shatter them like earthenware.’”
We have learned of God's wrath on the nations before:
Isaiah 63:3c: “I also trod them in My anger And trampled them in My wrath; And their lifeblood is sprinkled on My garments, And I stained all My raiment."
Further, this same imagery was predicted in Revelation 14(:17-20), where the anger of the Lord would be poured out on all disobedient nations and peoples — not merely Jerusalem. Leading up to verse 19, verses 17 and 18 are a grotesque description of the destruction of the ungodly. Christ and His armies will conquer, just as the prophet Ezekiel prophetically spoke against the nations of the earth plural (referred to as "Gog and Magog" in the chapter):
Ezekiel 39:4, 18a: “You will fall on the mountains of Israel, you and all your troops and the peoples who are with you; I will give you as food to every kind of predatory bird and beast of the field... 18“You will eat the flesh of mighty men and drink the blood of the princes of the earth"
Here, in Revelation 19, the judgment is against the nations, where Christ will have the ultimate victory over the kingdoms of the world.
Thus, the passages in Revelation 6 and 19 are quite different. The first appears to refer to Jerusalem, while the second refers to "the nations" (more specifically, Rome) who are both the objects of God's wrath.