Those transmitters just switch between "on" and "off". I wonder if your transmitters are all on in their default state? Maybe the the transmitter can talk to its own receiver because the signal is so much stronger. Make sure you have the transmitter pins programmed as outputs and forced... um.. low I think is the off state.
A bit of a long shot, I believe those transmitter receiver pairs are hand tuned. They might be on slightly different frequencies. Swapping around the transmitter and receivers should tell you that. But from your test of severing the transmit connection on the second unit, I like the first hypothesis better.