I did it on my old cap232. But i have 10sx. I had to do 2-3 mixes. Elev has a servo each side in sep channel. Mix both elev channels to ail in 2 separate mixes, you can change the directions in the mix, use the flip sw upper right to activate.
That's the rough and dirty version, I hope that helps.

I re-read your question- do you want to disable ailerons when you switch to tailerons? That would be much different. I'm clueless. Should be do-able, I'll have to monkey around with the tx to figure that one out. I like a challenge.