i don't think that's possible(or practical) as the game treats every unit separately. There could be a way to select targets, then give a "master fire" order, that makes every unit fire one after the other(much like arty, spotters and airborne units)
A way to avoid having everyone op-firing at the same unit would be: fire, use another unit to fire(no matter the target), then keep cycling through your units as you fire, until you come back to the first one, then start over. That's how i do it, it's a pretty fast way to play.
