The short answer to your question is, you cannot do it in the radio. The capability simply isn't there. You'll have to use a reversing Y-harness or other electronic aid to tie two servos into one channel. As you discovered, you CAN use a Pmix to make it happen, but then you have no trim on the slaved side.
If you'll look a few posts down from your original one in that forum, you'll see a fellow with the same question. He was asked about his set-up, etc., when a simple "no" would suffice. Perhaps "they" are reluctant to admit to any faults; dunno why that would be, because the 7C is a good radio otherwise. Why Futaba left out dual elevator servo capability on it is puzzling, but the fact remains.
Bottom line is, you've got two choices; employ an auxillary device to gang your two elevator servos into Channel Two, or get another radio. The fellow mentioned above decided to upgrade to a 9C.
Who won that one???
