High Current PCB Design Layer StackUp
Currents will be the same on both layers, assuming you only have a single supply. 

The bigger issue is making sure the motor connections / driver is connected really close to the power connection on the board. Do not have them on opposite corners..... I might even suggest going as far as routing the motor power and return using traces on their own and tieing them back at the power connector so they do not run through the power and ground plane. And yes.. heavier copper is better.

Further, if there are control signals coming to this board from elsewhere you may have issues with the cabling and ground level shifting between them that can mess up the logic levels.