If Tidus joined the party in FF X-2, then the whole story would have to be rewritten to accomodate his presence. I think that would have been too much for one disk to handle. If anything like that were to happen, Square-Enix probably should have made FF X-3 or something.

I never got the ending where Tidus and Yuna are reunited. I didn't like this game well enough to try.