I assume you mean that you want one of the thumbsticks to control each side of your robot.
Short version, the USB Dongle must be set to Mode 1 and have the xbox controller plugged in. The code will look similar to:
Code:
pwm1 = p1_x; //Right Side
pwm2 = p1_y; //Left Side
That is assuming that the dongle is put on port 1 and your drive motors are pwm1 and pwm2.
Take a look at the user guide for the USB Dongle
here Page 5 should show you what the code will reference. Look at XBOX/XBOX 360 Mode 1. Or if you need to still use the x axis on the controllers look at Mode 2.
I hope this helps.