Change Gain Levels is not a realy good solution.
1.) If you change the Gain Levels on linked Channels you have different Input Levels on the Dynamics - This makes no sense.
2.) With different Gain Levels you have this "Panorama Setting" at the whole System. Any Monitor ( In Ear System ) have this Stereo Image and can not make the own. Thats not pretty good.
Panning is indeed symetric, but it works L-C-R. Yes but it is not the same and sounds very different.
I´m not a Fan of B ehringer Equipment, but the B ehringer X-Air Series is an example for a very nice solution how to work with linked Stereo Channels. There is a linked Channel with a little double Fader and an arrow in the Label-line to expand it to normal Fader size. Then you can set an offset and after tap again at the arrow to switch back to the Stereofader, the Fader members and show this offset in "one" Fader - very nice solution.