I have a mouse with two thumb buttons... What I ended up doing was mapping one of the buttons to CTRL and one to ALT (or SHIFT... I can't remember). Then in IS Boxer I mapped melee strafe right and left to CTRL - RIGHT and CTRL-LEFT and ranged strafe right and left to ALT - RIGHT and ALT - LEFT.

That way, I can move the tank by not using the modifier keys, move the casters and melee independently, and holding down both thumb buttons allows me to move both groups at once (though not the tank)