Front calipers on a rear rotor
 

Could you use the front calipers on a the stock rear rotor?

The big question is why?

It's for my cart project. Two piston calipers are better than one piston. Better stopping power and I can't fit the front rotors only the back

Calipers are made for a certain size of rotor, so the swept area of the rotor matches the curvature of the caliper. Rear rotors are much smaller than front rotors, which makes their curvature different.

You would have to check whether it would work with a fitment test.

You may be surprised how well just a rear caliper will work. Once you lock up the rear wheels you don't need more stopping power anyway. This is why front brakes are extremely important. When you hit the brakes the vehicle leans forward, providing more traction to the braking tires.

James