Tosafos asks, why should there be a difference between 2 who agreed to go to court before 2 and 2 who agreed to arbitrate before 2, if in both cases, there was an agreement between them to go forward in front of two.
He answers that they agreed in each case to do it "how it's normally done". I think Tosafos means (this is the way I'm reading in Artscroll's notes as well) that since Din in front of 2 is not the normal way they can be Chozer.
Now my question is, doe this mean they can be Chozer and bring they're case in front of 3? If it means in front of 2 again, Ein l'Davar Sof!
Yitzchok Weiss, Brooklyn
I understood Tosfos to mean as follows. The scenario where Pesharah (arbitration) has a greater power than Din is where the two disputants came to two potential judges and said to them, "Do Din for us in the way that Din is usually done, or alternatively do Pesharah for us in the way that Pesharah is usually done."
Therefore, the two judges have a choice. If they decide to do Din, then since the directive was to do it in the usual way, this means that the Din must be done with three judges; so the intention of the two disputants must be that the two judges should add another judge and do the Din with three. However, if the judges decide to do Pesharah, they need not add a third, but rather the same two can do Pesharah. Therefore, Pesharah has a better power than Din because in the above scenario only two are required for Pesharah.
It is true that if the two judges mistakenly decided to do Din with only two, the disputants may be Chozer because the judges did not follow the instructions of the disputants. When they are Chozer they may do what they like, because it is as if nothing has happened yet. The disputants could say again exactly what they said the first time ("Do Din for us...") and we could start again from square one.
Kol Tuv,
Dovid Bloom