Maybe using the MONO/Stereo switch I mentioned, or if the unit allows panning the input to both channels.

Since you are only recording a MONO channel when one line is connected, why would you want to record stereo in that scenario?
Just wasted track data duplication.

I'd use MONO when one cable is connected and Stereo when two cables are connected.

FWIW, your piano will likely sound better in stereo, using both L/R connections in the long run.

If you are ending up with a stereo track and sound on only one side you may need to simply change the target track type to MONO instead, so it sounds centered (or panned wherever you want).

Knowing which program would help too, as RB also allows you to decide the track format, so if you are recording MONO you can record on one MONO track, and when using stereo you can record to either a stereo track or 2 MONO tracks.

In BiaB I'm not sure how much of the above is true.

I do not work here, but the benefits are still awesome
Make your sound your own!