Matt,

My guess would be that his transmitter was slightly mistuned and your RX was too, and his transmitter basically took over your RX (your RX locked onto his TX more likely). I do not think it would be possible for a second PCM TX to take over PARTIAL control of your aircraft - sending commands to some of the servos, but not all. That would take some pretty fancy synching up to replace/modify some of the bits in a series of packets, but not others. Seems highly unlikely...

Bob