Why do you need a different mod for single cell and dual cell cars?

Also, has anyone measured the max current available to the motor with this mod? I would like to do this mod with a dual motor mod, where it would make a HUGE difference. If my 2.6's can each draw 200mA, but a bit can only put out 275mA, that's not good. This mod is absoluely necessary with 2 motors, or else it's like 1.375 motors!