Why say 少说两句 to break up a fight?
「兩」 here is basically a synonym of 「幾」 (several). 「少說兩句」 means 「少說幾句」, that is, the argument arose out of speaking too much (more likely to drag up disagreements and past grievances), so speaking less is a way of not adding fuel to the fire.
It's not really a way of breaking up the fight, but a way of calming the situation down.