The manual isn't very clear on it from what I remember. I haven't had my DX-7 for a while now but I think you set up custom mix #5 or #6 with the elevator as the master and a spare channel as the slave.Just plug your second elevator servo into that spare channel. You may have to reverse it. You have to use mix 5 or 6 because those two allow the master trim to work the slave trim. The others have independent trim if I remember correctly.

Like I said, it has been a long time so I might not be 100% correct.. I seem to remember posting the procedure here a while back along with pictures of the menus. i will try and look for it.<br type="_moz" />