That's exactly what I've just done (as explained on p.62 of the RDS8000 manual). No problem at all. The first step (as explained by gravityking above) is to go to the ETC. menu, then down to D-EL ("dual elevator"), which is by default set at INH ("inhibited"), then using the "yes" button change this to ACT ("activated"). Then press the END button to get back to the main menu. That's it. The radio is now set up to use channels 1 and 7 for the two elevator servos.

The lead for the left elevator servo goes into channel 1 (the usual elevator channel) and the lead for the right elevator lead plugs into channel 7. You can then reverse the left side on the usual elevator channel menu and reverse the right side under the channel 7 menu. You have the full set of adjustment controls under each of these two channels.

*****

I just completed the preliminary steps in any radio setup:

1. Connect all the wires. (I also labeled the ends of all leads since I'll be detaching the tail for transport.)
2. Remove all the servo arms (which were just temporarily in place).
3. Turn on the Tx then Rx to allow the servos to move to their base positions.
4. Re-attached the servo arms, focusing on the best possible position (usually but not always 90 degrees).
5. Check that the servos are moving all the flight surfaces in the right direction.
6. Finally, adjust the push-rods for the closest possible mechanical fit.

Once all of that is done, I can move on to setting and fine tuning everything else on the Tx.