Two Mercs Build a Weapon

Mercs seem to build their own weapons a lot. The Engineer and the Medic invented their weapons in their Meet the Team videos. Although the Demoman received his unlockable weapons from TF Industries as a bribe, he describes bomb-making in his MtT video in some kind of bomb-making lab. It's implied in the design and some concept materials that the Pyro built its own flamethrower. The Soldier built the Beggar's Bazooka. Even the guys who didn't build their own doodads (the Scout, Spy and Sniper all explicitly purchase at least some of their weaponry from catalogs) presumably have some understanding of the technology... or do for the purpose of this thread.

What would happen if two mercs collaborated to build a weapon?

Design a weapon that two mercs would build together. It can be unbalanced. It doesn't even have to fit an existing slot! Throw standard design practice out the window. Is it a weapon? Is it an objective, like a bomb cart or briefcase? Is it a map feature? Give it some backstory.

Two Mercs Build a Weapon