It depends on the level. Sometimes, fighting someone 2v1 will make the other person kite you around gaining AP and you have to be able to cut them off which is sometimes hard. On smaller levels, I 2v1 to make the game end faster. Screw honor, we earned this 2v1 advantage.
There are some times where 1v1 with 1 spectator is better. On bigger levels, you wont want to chase them all over. Also, if you can generate AP yourself (PaRappa, Toro, AP generator at Alden's Tower) then I would do that while they 1v1. They either come fight you to stop you (success) or they leave you be and you get free AP to help your team mate (success).
You should never just watch and taunt. Keep the guy close to your team mate or try to protect him from dying. Don't interrupt your team mates combos, but if the player runs away from a team mate that is man-handling him, hold him up for your team mate. If your team mate is struggling, either switch out or give a little assistance. Idle players are useless.
Interesting, I actually hadn't given much thought to allowing 1v1 in 2v2 if the other partner is out.
Though, just tonight I was doing quickmatch, and was paired up in a 2v2, it was me and toro, vs jak and someone else. The partner for Jak never spawned, so right from the start it was 2v1.
I jumped in and started fighting, and the toro just ran out of the way, didn't sleep or anything. A couple of times I wasn't sure if he was actually there or not, but if the fight got too close, or the stage pushed him around, he'd move somewhere else. Jak also left him alone, and never attempted to use a super on him. I won, but it still felt more fair.
I may let a 1v1 happen next time i'm in a 2v1, as long as the other team is playing with good sportsmanship (no spamming, no teabagging, etc...)
if the other guy's partner died leaving just him to face me and my partner, yes i let them 1v1. but what i've been running into more and more recently is they not! i'll be in the corner or something so the two can go at it and here come numb $h!t running towards me while my partner beats them down.
hey they wanna pay attention to the one letting them 1v1 rather the one attacking them by all means. its your lose kid, that's all i gotta say.
I sit back and watch if the partner disconnected, died too soon, or ragequit early into the game, If they were actively playing and died off over the course of the game I'll just keep attacking the remaining player, I just want to wrap that game up already.
I make a special exception for Dante or Big Daddy, if his partner isn't there I back out and let him have a 1 on 1 even if he has three lives while we only have one. Those two characters are nothing more than a punching bag when ganged up on.