I've repaired ballasts before that were either mine, or an acquaintance. I've never fully assembled one from start to finish, but like you mentioned, they're not difficult as long as you know what you're doing. If you have doubts, though, I would advice purchasing a pre-built ballast. Electricity doesn't fuck around.

As for splicing leads to two separate sockets, I wouldn't do that. One ballast per bulb, otherwise you may be playing with fire. (Literally.)