I'm currently multiboxing 2 to 3 rogue's, with PVE mobs I almost have no problem in terms of positioning.
The fact that my first attack is a stun might give me an advantage but even without it should not
make it that much harder.

I usually start a fight, and run through the mob then turn around so you face him.With your 2nd char
following you he will most likely be in a very good position to engage melee combat. Additionally I
have bound some buttons on my mouse to make him turn left/right and go forward/backwards.

When fighting multiple mobs I hardly have any problems "setting up the mob" so the 2nd/3rd Rogue
gets a piece of the action. Just remember when the first mob dies, press the follow command position,
yourself and the mob in a way that your 2nd druid can turn do a short walk and have the next mob in
between your druids. And even while fighting, occasionally press the follow button to allow
him to realign so he will be facing you, this way the target will stay between your characters.

I must add that I find it very hard to fight underwater with the smaller mobs(mostly fish/eels).
The bigger mobs don't really pose a problem.

I'm not really gonna discuss what build your going to take, but don't let the positioning be the
decisive factor on it because once you get the hang of it its not all that hard.

For battlegrounds/arena its probably a whole different story, but in world PvP I haven't really had
problems killing people before the stuns ended, except for a 70 hunter who died shortly after.
Currently 63 and 62 and counting.