For the most part, the pilots can be almost shoulder-to-shoulder and fly safely. If there are going to be frequency problems, you'll get them when pilot A's airplane is close to pilot B's Tx when A and B are at the ends of the flight line from each other.

To get the possibility of intermodulation interference, you'll need at least 3 transmitters operating on evenly-spaced channels, such as 11, 13, and 15, for example, but 11, 22, and 33 could also cause a problem. If a LOT of people are flying, take any 3, and a possible problem could result.

Even so, this type of interference has been extrememly rare at flying fields. It's only a potential difficulty.

The formula involves any three channels: X, Y, and Z.

If 2X - Y = Z, you may have a difficulty. Any channel number can be used in any one of the variables. There is no order implied.

Again, this only dictates a potential, and not actual problem. Modern systems work very well in our R/C environment, and actual problems have been extremely rare, and usually with older equipment.