What Arduinos is this shield compatible with?For use with the Due or other 3.3v processors, you must configure the board for 3.3v logic levels. Find the set of 3 pads labeled "Logic". Cut the small trace between the center pad and 5v and add a jumper from 3.3v to the center.As of Arduino 1.5.6-r2 BETA, there is a bug in the Due Wire library that prevents multiple Motor Shields from working properly!
Arduino 1.5.6 is quite old now. Several years old, I expect. What version are you using now? Have you checked the GitHub bug tracker for this specific bug to find out which version fixed it?Cutting a trace on a PCB is more difficult than you expect. Just get a sharp tool like a knife or pointy scissors and gouge away at the track until you can see exposed copper on both sides of the cut and no copper in the middle. Making the new trace will require a soldering iron. Do you have access to one?