As others have mentioned set up some combination of keys for slave movement versus main movement (for myself I use the arrows for my main and WASD for my followers).

I can't think of any boss in UP that requires you to focus on two different targets, but anyway, in such a situation, you could use IWT (and CTM for melee). IWT is "Interact with Target" and allows you to set a target for a subset of your group (easily done with ISBoxer, don't know about other software) and have that subset attack it if it's a mob or chat/trade with it otherwise. CTM is click to move, which melee will need if they're to use IWT properly.

It also sounds like you're using a focus-led setup, and this has issues I think. A focus-less setup is much more versatile.