Play Isaac as the Duck Game duck: bullets instead of tears, and weapons on pedestals pulled from your own copy of Duck Game.
In your The Binding of Isaac (Repentance+) run, Isaac becomes the Duck Game duck: he walks, takes hits, and quacks with his own sprites and sounds, and shoots bullets instead of tears.
How to play: Hit Play, choose Isaac, and start a run. The mod automatically enables Isaac mods if you had them disabled.
You need The Binding of Isaac: Rebirth (with Repentance+) and Duck Game installed. The duck, its weapons, and its sounds are pulled from your own copy of Duck Game the first time you play; the mod doesn't include any files from either game.
This is single-player. The other Isaac characters are unchanged.