Just happened to me the other day. I get a call from the cabin while holding short for takeoff. Two passengers took their mask off and were refusing to put it back on despite two PA's from the FA. The other passengers around them were getting upset and rowdy. I set the parking brake and luckily the FA did an amazing job at discreetly going back there and getting them to comply before tensions escalated and without the need for us to intervene from the cockpit or worse have to return to the gate.