The two most common frequencies (in the US) for RC toys is 27MHz and 49MHz. If you have one of each, all is well. If you have two toys that operate on the same frequency then you're going to have problems with radio interference or crosstalk.
When shopping for multiple RC toys, look for toys that come in more than one frequency or toys that have band selectable frequencies. These usually have a switch that lets you select Channel A or B or C, etc. and the package will often indicate that you can choose multiple frequencies so that 3, 4, or 6 cars can race together or other similar wording. RC toys that come in sets such as the Air Hogs Havoc Heli Laser Battle Set, Bump 'N Chuck R/C Bumper Cars (Buy Direct), and Fast Lane Radio Control Thunder Speed Boat Twin Pack (Compare Prices) are designed with group play in mind.
Earlier this year I gave thumbs up to Jada Toys for the helpful labeling they put on some of their toys. ("This vehicle operates on 49 MHz. Can only race against 27 MHz vehicles.") More labeling along those lines plus careful attention by consumers would prevent disappointments that arise when buying multiple RC toys with the same frequency.