Note- if you are experiencing audio in one output channel only when using the CAMERA APP, please click here. The article below is for GARAGEBAND.
If you're only hearing sound from the left output channel when recording with ONE for iPad & Mac or Duet for iPad & Mac in Garageband (or other apps), this is most likely because you're using a stereo track when you need to be using a mono track.
If you create a MONO track in Garagband, it will send your input out both Left & Right output channels. When you create a stereo track in Garageband, it will hard-pan both inputs, so input 1 goes out only the left and input 2 goes out only the right. This is why you're only hearing sound in 1 channel. Stereo tracks are typically used for stereo mic situations or if a keyboard has a stereo output.
Here are some videos on how to set up ONE with Garageband (you can also apply these videos to Duet):