Hi N1EL,
With the awesome help from @jfjoubert, we were able to come up with this code for lua. I use this on a CH Throttle Quadrant and have 1 lever for flight idle to max throttle, 1 lever for flight idle to reverse, 1 lever for hi/low idle to cutoff, and 1 lever for flaps. It's a bit complicated to set up so just fire away. You will need to set your throttle lever to Collective, idle-to-reverse as Wing Sweep, hi/low idle as Prop 1, and flap lever to Mixture 2. Depending on your setup, you may need to tick the reverse axis box. Mine is ticked for Mixture 2, Wing Sweep, and Collective.
The way it works is that you need to bring your throttle back to flight idle before the idle-to-reverse lever would work, but regardless of the idle-to-reverse lever, the throttle would work if you move it. I think you need to be on flight idle on the throttle too before the hi/low idle lever would work but I don't recall if I tested this. Obviously once the hi/low idle lever is set, I don't touch it again until shutdown.
Hope that helps!
Mixture Separate Axis.lua