The RS 120 Wireless Headphone System has a transmit range of approximately 300 feet and there are three (3) channels (frequencies) that the transmitter can transmit on.
It is possible to use up to three (3) transmitters in proximity of each other and this can be done without the transmitters interfering with each so long as each transmitter is set to a different channel (frequency).
On the front of the transmitter near towards the bottom is a small white switch which is the CHANNEL switch. This switch has three (3) positions (left, center, right) and each position corresponds to a different frequency.
When setting up multiple transmitters make sure the CHANNEL switch is set accordingly on each transmitter. For example on the first transmitter set the channel switch to a position (ie: to the left) and on the next transmitter set the switch to a different position (ie: in the center or to the right).
Once the transmitters are set accordingly you then use the TUNE dial on the headphones to tune into the appropriate audio signal you want to hear.