RE: How close can transmitters be
The actual formula is:
(2 x (channel A)) - (channel B) = (channel C) or (2 x (channel B)) - (channel A) = (channel C)
This is called "3rd Order Intermodulation" because a 3rd frequency is being generated. If someone is flying on channel C, and their receiver is closer to the channel A and channel B transmitters than to its own transmitter, a possibility of it getting interfered with exists. This intermodulation is quite weak, and the channel C receiver would have to be very close to the A and B transmitters and rather far from the C transmitter to be affected.
This isn't common at all with modern systems and the fact that the flyers are all in one spot. You'd more likely get it if two people were flying from one side of a park, and someone else a few hundred yards away at the other end of the park had the channel C rig, and their plane was close to you. Then their receiver could get "hit".