Mac Forums

Please help... I've been using iMovie, and don't get me wrong, really cool program but I want something different. I like making amv's when I have time and I've recently seen an effect I like, err, it's when you can overlap clips so you can make it look like two characters are talking, when they really aren't. Does anybody know of a program that can do this??? Or what this effect is even called?? [I've heard of final cut pro or express or something like that, but it's very expensive, and sony vegas doesn't come for mac from what I've been told.]