Hmmm, this leaves me at a stump. I find it hard to believe that MX fused with Terra as two complete human beings because it seems... I don't know... a bit difficult? A lot of things could go wrong but I don't know enough about MX, regarding his personality, to talk of what he would personally choose to do.
I think, maybe, now that I think of it, that MX himself did something to Terra to force Terra to cast off an Unverse so he could fuse with Terra & continue living for whatever reason. So, like, halving Terra so he could fit himself in & use Terra as a host, almost? Sort of like, unlinking Terra's memories so Terra would forget everything and hence wouldn't resist whatever MX wanted, and maybe MX did something during the fusion, like... put his scientific knowledge (if he was a scientist) in place of Terra's memories, which could explain why Xehanort is so intelligent. I don't know. I'm grasping at straws now and not really believing myself.