Rather then building a macro for dps like these:

/assist Tank
/cast Something

/cast [target=FocusTarget] Something...



You could go with straight spells for the DPS.
And a specific key you press for Assist, or FTL Assist or whatever.

That way, whenever you want a target switch, you press the assist macro.
But until you do, the other toons stay on whatever they have targeted already.