Not really the answer to your question, but an alternative solution, which has proven to work in my case.
We were working on a line-following robot with an ATMega 328P (Same as arduino, but withouth shields) and thus we wired an shield directly. For the shield we used, we only needed 4 inputs (pwm left, pwm right, direction left, direction right). Try to find out what your shield is using, on which pin, and you can drive the shield directly withouth stacking them (as they then would use the same pin).
My alternative solution would be to use 1 driver. And wire the motors parrallel.
If you choose to use 1 shield, you can stack it. But for other shields you have to check which pins to use. There is no drawback on using one shield, the only thing is that the motor driver will draw more current, but they're basically designed for this.