OK, here is what I've been doing...

1- From the main menu where you program the name of the airplane and stuff, disable the switch for the slave channel you are intending to use. In my picture I have both the flap (aux-1) and aux-2 channels set to INH since I'm using both dual elevator and dual aileron servos. Doing this insures that bumping the switch won't affect one of your surfaces.
2- In the regular adjustment menu turn on mix #5 or #6. Select the elevator as the master and the desired channel as the slave. In this case I am using the flap (aux-1) channel for my second elevator servo so I have flap set up as the slave. Make sure you set the mix percentage in BOTH directions. In my case the linkages were dead on so 100% in each direction did the trick.

If you need to reverse one channel, just do it in the regular reversing screen. You can also control end points for each elevator channel separately to fine tune everything althoug I'd do it as best possible mechanically first.

This may not be the correct or the best way but I know it works.