[cbetz]

opposing barrels fired simultaneously might work, though it would be a bit wasteful.

[Symmetry]

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

[pilgrim689]

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

[MiguelHudnandez]

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

[pilgrim689]

"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?