[Al Skierkiewicz]

In all liklelyhood you have them wired to run against each other and you have not calibrated the speed controllers that feed them. Under this condition, one or both (if both are hot) are trying to move because of the mis-calibration but are being held by the opposing motor.
I hope that they are not connected directly to the battery or the output of the main breaker as this is not allowed under the robot rules.
James, you can use a "Y" if you reverse the output wiring of one of the speed controllers.
The best way to check operation is to remove the coupling so that each motor can run independently, then power up the robot. You should see an effect immediately and I am guessing they will run in opposite directions and speeds. Reverse the wires to the motor that is running backward when you push on the joystick, then check calibration so that don't move when the power is on but the joystick is not pushed. When all of this is working, then check operation to see that they are running at about the same speed. If all of this is OK then install the coupling and check operation again.