ZaCloud is not some alternate being. It's Cloud using the mannerisms of this successful, impressive guy he wanted to be like.
Furthermore, Aeris actually states several times she isn't falling for Zack in Cloud. If anything, it's quite the opposite. The similarities don't turn her on, they bother her. She also states that they are different, which means Cloud isn't stealing Zack's personality. Since Aeris knows Zack the best, she knows what parts are his and what to separate. The similarities of their meeting brought up the old wound and made a link between their stories, but she makes it pretty clear she doesn't want a replica of her ex-boyfriend.
@Legretta: I don't think they ever got far enough to be in a relationship, however, I am of the belief there were mutual feelings involved. Really, if this story is telling us Aeris views him as her lover, I don't see the need to debate this. I don't see why they would purposely use this word if they weren't, as Aeris isn't a delusional person. If there was no indication of a mutual romantic feeling, then she wouldn't see him this way. And as I stated above, since she is rejecting the Zackisms, I don't see this "ZaCloud" as an excuse.
edit: fuck I type too slow.