Racing DigiQ

Hey guys. I Just got my digi q last week and I registerd for this sight last night. And ive got a question mabie one of you could anser. My instruction sais that you can only controll one car with one radio channel and the cars come assighned with a radio channel. I want to race my friend but how do you tell when youre buying which car has witch radio number.Or can you change the controller number after you buy the car. If one of you guys could help me it would mean alot.. thanks

-nightrider