If you meant that you want to have M1 and M2 respond to the same command, you can do this by converting the controller as a single channel model. The v1.9b manual shows you how to do this.
After this modification, both channels will respond to commands sent to Motor 1.
However, after this modification, it is not possible to give separate commands to each motor.
When operating the controller in a dual channel configuration, there is no command that will change both motors at once. You will need to send two separate commands.