Are you using windshield wiper motors or some other type of automotive
motor? Is your chassis metal(conductive to electricity)? These types
of motors and others are grounded to the body of the motor. So, when
you run two of the motors at different directions you will have a
short across the body of your robot.
If this is the case you will have to find a way to electrically
insulate the motors from the chassis.
I built a sumo robot a few years ago that used windshield wiper motors
for the drive and had this problem. I ended up mounting the motors
with nylon screws and rubber washers.
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