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Pressing two buttons at the same time shouldn't be a problem, it's if you have it so it presses one button then another button where it becomes a problem. If you bind 4 to send whatever key moves forward and an instant cast spell it should be fine. You can't use a non-instant since the movement would stop you from casting it.
Bind the key you use as backward on the main to forward on the alt, makes it a doddle for simple combat positioning especially for melee. One quick tap will have both characters standing at the same point. :D
For a warrior, bind the key that initiates charge to forward/backward on the alt. You tap it on the warrior and the shaman stops following.