If it is the same transmitter on both planes and the batteries are in good shape it is possible to get radio intereference from a number of sources, not only rc transmitters.
For example, I found out the other day (the hard way) that a police radio tower near the field will interfere with channel 11 & 12 on occasion.
And just because you can't see someone else with an rc transmitter doesn't mean someone in his back yard somewhere isn't causing the problem. The range on most transmitters and be up to a couple of miles.