This is a well understood problem: http://en.wikipedia.org/wiki/Recoilless_rifle

I don't think so... Where is all your extra energy going?

In roughly equal and opposite directions. You'd just need something between the two weapons to withstand the compressive force.

"In roughly equal and opposite directions" sounds like you think energy cancels other energy like opposing forces do. The energy needs to go somewhere; it can't bump into MORE energy and disappear. What would this "something" that absorbs all of it be?