Man, PC-on-PC fighting is THE BEST. As the MC you just get to sit back and watch the show, cause that shit is *always* interesting. I've found PCs love to get hit by the other PCs cause then it gives them +Hx with the person who hit them FOR EVERY SEGMENT OF HARM. Shoot somebody with a shotgun? It's almost like giving them a free experience point! Also, you really don't want to give your rival too much Hx with you, cause then they can interfere with you when you need to do shit the most...
Also, there's a HUGE incentive to fuck with each other as PCs because most inter-PC moves don't force the 'losing' PC to do much, but they *do* mark experience if they get forced to do something. Between the xp and the consolation prize of Hx, intra-party conflict is hilarious, fun, and enjoyable even for those who take a bullet to the face.