I'm pretty sure you can't program one steering wheel control to do two functions. I was in the same boat with my JVC install. But I never go "backwards" when listening to songs or the radio. So I programed my back toggle to go forwad though my radio station list. So up skips songs and tunes the radio depending on mode. Down skips preset radio station. Can be confusing if someone else is driving my car ... but why would I care?
If I need the back functions, I use the head unit. Just another suggestion.