I think it could happen. They have "worked" together in the past (not directly) and they are bound to cross paths whilst promoting their albums this year.
GaGa has praised Britney many times in the past and Britney recorded "Quicksand" which was written by GaGa. They have both worked with the same directors (Jonas Ackerland, Francis Lawrence) and the same producers (Darkchild, Fernando Garibay).
No. They don't need each other to succeed, they're both gigantic stars. And collaboration of 2 huge artists is not equivalent to huge hit: Videophone, Dirty Picture, Somebody to Love, etc. etc. They would waste a lot of resources if this doesn't smash hard.
