When more than 1 transmitters are used in a transmission channel, transmitters influence eachother in this sense, that one transmitter intermodulates the other and visa versa. This can cause side frequencies that are equal to frequencies of other transmitters in that channel. One has to follow a very precise frequency scheme to fore-come this. This can be calculated with calculators (excel), but it is often done by the manufacturer by means of preset frequencies. It is wise to use these presets.

Here’s an explanation of what causes intermodulation.

Here is a calculator