That would be true if you were dragging the wheel over the surface. However, the wheel is rolling, so the friction force you need to model is rolling friction (plus losses in transmission) which is generally a lot less than sliding friction.

Also, using "2" is confusing, because that's a hard-coded constant based on your number of motors (which is 2.) Using "Nmotors" or doing the calculation for the sum, and later dividing by number of motors, would be clearer IMO.