to continue using two ninjas for the hope of bypass does increases the overall chance of bypass
Say both are at 5% bypass chance and you have 1 ninja and 1 crit troop. each tile has a 50% chance of being the ninja and then those 50% of the tiles have a 5% chance to bypass. But if both are ninjas at 5% then all tiles have a 5% chance to bypass since all tiles will become ninja troops.
SO technically more ninjas means more chances to bypass and when it does bypass the attack of every hero of that color is used in the damage calculation. But it does not increases the percentage of any given tile to bypass.