Dual or triple rates do NOT take up a channel. Rates simply allow you to assign different percentages of travel for a selected channel. You can set dual rates for all your control curfaces. Same area of the menu you set exponential which contols sensitivity around center. You can assign which switch you want to use for rates or whatever. Switch down 50%of servo travel...switch up 100% of servo travel...just an example not a suggestion of the rates you set. Get some help from one of those fellows at your club with a Futaba computer radio you mentioned.

A good bit to learn when you get a 7 channel computer radio, 9 even more, 14 more again. More channels more to learn. They come with a large comprehensive manual.

Addional updates and additional programming examples available at:
http://www.futaba-rc.com/faq/faq-9c.html look at question #24 for this answer.
That site is for the 9C.
Put 7c in the place of 9c in the address and you have the site for the 7C.

Have fun.
Michael