I bought both of my sons a remote control car one is a buggy, the other a hummer. one controller controlls both cars so they cant use them at the same time. is there a way to fix that?

You can change the crystals over in the car & transformer,problem is that they may be soldered into position & would need to be unoldered & new ones soldered in. Can get them of of ebay pretty cheap. Should say what frequancy car is on underneath & on box. Some controllers have a switch & can choose out of 3 frequncies.Hope this helps on deciding to go further.