Yes, that should work just fine. Just load the game, and when it says "waiting for players" press escape to open the in-game menu, then use Manage Players to disable the second player slot. I think you have to play the player in the first slot, iirc. But I could be wrong.
And then when you want to play a game (any game) in multiplayer just load it, pause, open the in-game menu and manage players and open the slot and re-sync. It will then say waiting-for-players and your friend can connect like usual when connecting to an mp game.
Probably want to keep the original save around in case your friend doesn't like what happens