I currently discount the possibility of aft tubes because they would've been used in "The Changeling".
...Because the target is at "123 mark 18", past the 90-degree mark?
But that is before
Kirk tells Sulu "That's our target". Sulu could well have pointed the ship towards said target there and then, especially when subsequently hearing that Kirk wants torpedoes used - why not point six or more bow tubes at the baddie, rather than just two stern ones? It is only after that, with the third attack, that Scotty specifically says that the ship has lost "warp maneuvering power", and even that shouldn't have to mean that Sulu would be unable to point before shooting.
Kirk only defines "torpedo two" later in the game, potentially after Sulu has finished turning. This could thus easily be the same "torpedo two" that Kirk specified as part of his forward spread in "Journey to Babel".