If you only use one mic and record in Stereo mode, it will only record on one channel:
Mono Mode: Both tracks of the RX output the same auidio which overlays the audios from the TX1 and TX2. When connected to cameras, sound cards, or PCs to conduct live streaming, you are suggested to use the mono mode.
There are always two tracks in whichever mode.
Safety Track Mode: The left track of the RX outputs the overlaid audios from the TX1 and TX2 while the right track of the RX outputs the same audio but with a -6dB gain to avoid sound overload. When connected to professional devices such as cameras or recording pens, you are suggested to use the safety track mode to avoid cracking to a large extent and for the convenience of conducting afterward modification.