It could happen. Not saying it will. But as the others have stated, there are elements that indicate the possibility.
I'm still not sure how I feel about it tho. One one hand, new Keyblades! Coordinated with the character's design! Perhaps allowing them to use more rad attacks in a fight!
On the other hand... new Keyblades. RIP diversity. And maybe instead of helping them stand out more, the battles would feel very samey.
Now that's an interesting thought, and it could happen, given how Ven's heart was doing basically the same thing in providing Sora and Roxas with the second Keyblade.
I'm not seeing it though. Vanitas with two Keys sounds just... off.