Psalm 42 and 43 in English translation are separated following a tradition of the LXX but are translated from the MT apparently being a single composition via all the commentaries. Can you offer a proof of them being different psalms?
Although the two psalms do seem to have been one at some point (due to the refrain), the MT does in fact separate the two psalms. You can see the Aleppo Codex here:
Or the Leningrad Codex here:
In fact, what's preserved of the Dead Sea Scrolls in 11Q Psalms (photo) might even show the beginning of Psalm 43 starting a new paragraph:
The Jerusalem Talmud (Shabbat 16:1) does speak of 147 psalms, which might indicate that some of the 150 psalms in the MT were originally combined. From the Babylonian Talmud (Berachot 9b), it seems that the first two psalms were originally considered one, but identifying any others as having originally been one is a matter of speculation. Psalms 9-10, for instance, form a broken acrostic, and might have originally been one, but there's no question that the division of those two psalms, like that of Psalms 42-43, was present in the MT and its predecessors for a long time.
In the Salter psalms that do not have a prescript are often ment to be read together. Psalm 42 begins the second book of the psalter is is part of an elohist psalter of about 40+ books. Since the LXX divides them it points to the division being very early. The reason is to make a collection of 42 elohist books.