Which classes and levels does your toons have?
You can use assist and /ui action toolbarSlotxx to fire action but you won't be able activate autoattac without pressing the key. Run both Instances in windowed mode, place them near each other and than just switch one per ememy to activate autoattack.
If you have a bit of experience in programming you could send keyboard/mouse events to one of the instances...
With a second PC it would be easier