I do wish I could help out with this, but I have never played with mixing because I have never needed it. I did try to go into my radio and play with them, but couldn't set up the mix to disable via a switch. I know it is possible, I just don't know how.
Now that I type that, try this... Setup one mix where you want it mixing and on. Then, back out and set up another mix, on the same switch and move where it is activated. I think this might work.
If you can, pull your model out of your Tx and put it in a .zip file and send it to me. I will plug it into my Tx and see if it works since I'm sure what I said above is confusing lol.