You'd need your software/hardware to mouse broadcast.

In my case (IS Boxer), I add my toons to an Action Target Group.
I've called this one "AoE on 4".

On my "4" DPS Key, in addition to the FTL stuff it does.
It also "sends next click" to all members of "AoE on 4".

So I get a targeting circle in every window.
And as I move the mouse around in the main window, the circle moves in all of the other windows too.
When I do a click, the click occurs in every window.
So Left click selects the area for the Treants, or Right click cancels the spell (same functionality here as one-boxing).