Well in my limited experience I don't think anyone controls both characters, since they will sooner or later be going in different directions no matter how careful you are, because of lag and other reasons, usually the others just /follow a main toon, which is why most people play casters or a tank/healer combo. If you have your follower set up to autoattack on /assist, he will melee if he gets close enough to the mob. With your main you just have to kind of walk past the mob and turn around to face it, and quite often the follower gets close enough and is facing the right way to get some hits in, but its not terribly reliable of course, which might be ok for a pally combo since your also doing some magic damage.

I have my tilde key run a /follow focus macro on my 2nd toon, and quite often during a battle if he is facing the wrong way but close nough for some melee, hitting this key will turn him towards me, and if he previously did an /assist focus attack on that mob, he will begin to melee.

I could be way off base though, since I havent been doing this very long. I played 2 ele shammies for a while, now playing a resto shammy with my 70 druid now that shammy has gotten close to 70 in preparation to dual box them to 80.