As far as making both balls in a combo, the ob1-to-ob2-to-pocket line has to be straight, otherwise ob1 is cutting ob2, and will move away along the tangent line instead of following. Also, ob1 has to be spinning (rolling) forward when it hits ob2 in order to follow. Some people think draw on the cue ball will put follow on ob1 because of gear action, but I don't think there's enough friction between the balls to really put any noticeable follow on ob1. Really what you need is enough cloth between ob1 and ob2 that ob1 picks up forward roll from friction with the cloth. As long as they're at least 2 feet apart, and you don't hit the shot super-hard, you'll get ob1 rolling, and it'll be a follow shot.
-Andrew