If you mean searching prior to lobby:
Emulated LAN. Make it automatically go through menus.
Players can't join the game even to observe once the game is in progress, so the watching can't be done.
If you mean in-game:
Add players to loitering player group on map start, remove upon leaving. Upon search, pick and remove two players from the group who then fight. At the end of match, add back to the group if the player is in game.