If ThinkTanks doesn't fit the bill:
I'm not going to watch gameplay or confirm coop, but here is a list of mech games that may be right. Just try hard to remember if you're mixing together details from other titles. Very few people have a perfect memory, and the more vaguely you remember something, the more likely you are to get even the most basic details incorrect. That said:
Battle Engine Aquila
Gun Metal
Gungriffon Allied Strike
MechAssault
Phantom Crash
None of them quite sound exactly like what you described from the vague and lacking Wikipedia descriptions, but the list of games with vehicular combat is already small, and the number of mech games are small enough to fit in a single not all that big Wikipedia article with every other vehicular combat game.
