Bunker Busting and armor

[Many Mariuses]

The discussion prompts a related question, which I alluded to in my first post. If you have weak armor units (such as the Russians in K42), should they combine their fire with other units in the hex, or fire individually?

I realize there are a number of other factors involved here. It seems as though generally, you should fire each unit separately, as it triggers more morale checks on the opposing unit (if this is incorrect, please let me know). On the other hand, I can see how at certain times you might want to combine fire, such as in situations where you want to minimize opportunity fire.

Still, in a situation where the firing unit is most likely to get a "No Effect" result, it seems more beneficial to combine fire with other friendly units to boost its effectiveness. Thoughts?