further BBS and secret movie spoilers! D: also days spoilers! plus speculation about kh3! D: D: D:
Well, in the beginning, Sora offered some of his light to Ven. Five years pass. Then Ven's heart comes back and shelters in Sora's heart, in a way that I took to be more similar to how Kairi's heart hung out there for a while. Ven's heart "goes to sleep", but we've made a fairly big deal in canon about how sleeping != death. Sleeping Beauty for example...! Sora explains how forgotten memories "sleep" without disappearing in CoM, and his memories of Namine "sleep" for the duration of KH2. Ven's heart is sleeping at the end of BBS -- before it goes to hang out with Sora, this is the terminology Yen Sid et all use -- but all the characters are confident it'll wake up again someday. Sora himself slept for a year without dying...
So there you go. (I actually had a similar fusion thought, and now I have talked myself out of it. SOB.)
The real question, of course, is how it's going to work in KH3, when Sora opens the door and Roxas and Ven become real again. People have wondered for ages how Roxas and Sora being separated was going to work, because omgn Sora is alive and Roxas, his Nobody, should be inside him...! But Days was pretty good at explaining that, actually, Sora DIDN'T need Roxas inside him until Xion started consuming his memories. Xion emptied out Sora's heart and then fused herself with Roxas, making it so that we needed Roxas to put Sora back together. The implication at that point was that Roxas and Sora could have coexisted just fine, and BBS, I think, solidified that and showed us why.
Basically: it turns out that at the end of KH2 Sora has gone back to having more people in his heart than he really should.
But while answering that question, our current scenario does offer up other questions -- how are Ven and Roxas going to coexist? And heck, how is Xion going to exist, period? (And is Sora going to bring back the replica Riku, too, which had equal claim on the right to exist?) And what's going to happen to Axel, when we bring him back? Is he going to keep being a Nobody, or is Sora just going to return his lost heart to him? QUESTIONS, BABY. QUESTIONS.