[Alan Anderson]

If you're putting two CIMs on the same gearbox, you can use a pwm Y cable to control both of their Victors from the same pwm output. Then just program it as if it were one motor.

Using pwm outputs 13-16 is fine as long as you don't have a lot of interrupt sources active (from shaft encoders, gear tooth sensors, fancy timers, and the like). If you have added anything like that, they'll act jittery and occasionally even make motors run when you want them to be stopped. You can avoid those outputs, or you can install Kevin Watson's fancy pwm support library to tame them.