Hi
Without know what hardware you are using I can only guess what hardware you have.
When I made a wheeled robot, I also used skid steering and I used an H bridge so one motor could reverse while the opposite side went forward.
I had 4 motorised wheels so it turned within its own length.
If you are using standard DC motors, you can use components to make a circuit or an IC like L293d up to 1 amp per channel.
If I remember correctly, to go into full reverse, you would need an extra output from the microprocessor for each motor.
You could try just turning one motor off, depending upon the surface, it might be enough to allow the other motor to drag it round.
I'm not an Arduinista which is what I think you are using, so I only half understand your code.