I wouldn't put one of the adaptors in there unless you figure out a button strategy. Some people use the Player start buttons so they don't have to hack the CP, but that can't work for P2 on a cocktail. I would sell it and get a different game / cabinet to work with. A multi-pac is great, but you can just put one of those in your upright. Plus the pac cocktails are the most sought after by regular folks and can bring in decent money.
Edit: Jr Pac if you get the field conversion board...