They come from different areas on the globe, and have different needs for their environment. Humidity being the main difference. In short, no, you should not ever keep leopard geckos and fat tailed geckos together.
They aren't similar really at all. They come from two completely different areas.
About the breeding, I heard somewhere (it might have been here, I forgot) that someone actually DID get fertile eggs from an AFT x leopard gecko cross, and managed to hatch them, but the hatchlings only lived for a few hours then passed away. I'd be curious to see what they looked like.
I think leos and AFT look very similiar also. But when you check into the housing details, they have slightly different needs, so should not be in the same cage.
It's amazing that nature made two lizards so alike, so far apart!
Unfortunately I see AFTs housed with Leos all the time at petshops. ::shrugs:: BTW, both the AFTs and Leos don't look happy or healthy when I checked them out.