I was having the same trouble on my 7CAP trying to program a second channel for second elevator servo. IT CAN BE DONE!!! The reply above was accurate, but let me add some details for those of us who are not all that familiar with the 7CAP and where some of the settings are.

First of all, you do indeed need to make sure that no switches are assigned to the channel that you want to use. My second elevator servo is on CH5. I found that CH5 was assigned to one of the switches. You can find these settings in what I call Level 1 of the programming menu. Scroll down to PARAMETERS and click it. NOW HERE's THE TRICKY PART. After you select PARAMETERS, you have to arrow down to the second page (who knew there was a second page?!). On mine, it showed CH5 and CH7 each set to a different switch. Cursor down to CH5 and Scroll the dial until "--" is selected and no switch. IF YOU DO NOT DO THIS, THE NEXT STEPS ABSOLUTELY WILL NOT WORK AND YOU WILL THINK YOUR RADIO IS BROKE AND YOU WILL SEND IT BACK. No need. Keep your fine Futaba 7CAP and follow along.

Now you can set up the P-MIX as mentioned above to make CH2 (elevator servo 1) and CH5 (elevator servo 2) work together. Go to the Level 2 programming (by pressing the Mode/Page button again). At the top of the menu is P-MIX1. This is the one I used. Select P-MIX1 and arrow into the part where it says INH and scroll it to ON. Note that there are TWO pages to this menu as well, so you may have to arrow down to locate the INH/ON section. Now arrow to the MAS> and set it to CH2. This will be the master channel. Arrow to SLV> and set it to CH5. This will be the slave channel. Arrow down to the SW> and set it to "--" . This will make the mix on all the time which I think is appropriate since you will always want the second servo to do what the first is doing. I imaging accidentally setting a switch on this and turning it off in flight could be disastrous.

Finally, still under P-MIX1, arrow down to the second page to the RT> section. There will be two 0% settings and one of the % signs will be flashing. I changed both of them to -100%. This tells CH5 to do 100% opposite motion of CH2. This was appropriate for me given my servo horn settings, but just make sure that however you set them that both elevators do the same thing going up and down. NOTE: In order to change the lower % setting, you will have to push down on the right stick to get it to select the lower setting. It took me FOREVER to figure this little trick out when I first got the radio, but this is common with much of the programming on the 7CAP.

So hopefully, this will help any other newbies out there like me.

P.S. - Dear Futaba, please consider rewriting your 7CAP manual. It is not good. Not for the uninitiated anyway. The manual I have does actually have "Dual Elevator Servos" in the glossary defining the term, but HAS NO REFERENCE TO A SETUP PAGE. Seems like this is common enough to require some direct explanation. No offense, just my not so humble opinion.