Yeah, lore wise once he returns Ven's heart he shouldn't be able to dual wield any more.
But I still loved the hell out of doing it. There are some neat substitutes, though. Can't remember the name off the top of my head, but there was one transformation in BBS of Ven where he got these light swords behind him like these giant wings. That form was my JAM. Loved it.
I have a feeling if Sora uses multiple Keyblades again it is going to be a very special reason tied to the story and probably wouldn't be accessible outside of the battle/scene it happens in. Sort of like, say, the Leviathan fight in XV with the armiger.