It is definitely functional, a bud and I Throne every day. Hope this helps!
1. Go to Settings, Game, Delete Data(NTT starts out with everything unlocked, so you need a fresh save)
2. Go to Play, and on character select, enter /savegame plain.sav (Note: Can be any name, though having a new game save is nice in case you plan on saving multiple files)
3. Start CoOp with someone, and on character select, enter /loadgame plain.sav
4. Whenever you wish to save, just type /savegame (name).sav