If all else fails, consider using a two-channel motor driver rated to handle your current requirements in its standard configuration. This will simplify your wiring, and reduce the likelihood of ground-loop issues.
I’ve used to MD03A from Pololu in a large robot before. Its a monster for the price. The only drawbacks are:
- you will need to make a wiring harness to carry logic signals from your microcontroller to the board (it’s not a shield). I suppose this isn’t really an issue because you’re already doing the same for two shields.
- There are only two mounting holes, so it’s happiest mounting on standoffs, using the header connection on one side as a support.
If you wish, you can close-the-loop using current feedback from the driver - this can be helpful to diagnose if your motors are stalling or failing.
If you decide to go for this solution, pay particular attention to the constant-duty current rating (infinite run time). If your motors are about 100W @ 12V = 8.5A then you’re about on the limit for the VNH3SP30 so it might be better to opt for the VNH2SP30 model, linked. Adhering a couple of small heat-sinks to the h-bridge chips never hurts too.
