Make the tray portion of pc-mixer also "L/R channel" aware. It reports whichever value is higher on the tray slider, but keeps the difference between the two channels the same when modified.
Now make sure that if the main Mixer is open, that any changes through the tray also reflect in the main Mixer. Now both the tray and main mixer UI should be in sync at all times (even though if the mixer is open the user will probably not be using the tray to change things).