The PC is a Bard 11/Rogue 1/Fighter 1. Her dueling weapon is a +2 keen thundering crystal echoblade. Her AC is probably in the low 20s and her HP at the mid 60s. She has the whirlwind attack feat chain.
So each is roughly 2 above what I calculated as being an "equal" encounter.
Now you need to factor in whether or not it is "lethal" or not and other "risk factors".
Are the paladins oath bound to not kill during this duel?
Is the bard evil?
If the bard is evil, I have difficulty fathoming a non-lethal duel.
It almost seems like you are using the KNight's (PHB II) code for the paladins.
It doesn't seem like they should have any qualms about combining forces against a higher level foe, unless it is of course a "contest".
Knights on the other hand must follow a very strict code of conduct for fights.
If the duel is non-lethal combat (from the paladins side) then it is a much lower "risk" encounter and the numbers become way scewed since the paladins are fighting at a huge disadvantage since there is a penalty to use a lethal weapon in a non-lethal manner.