Need a Way to activate secondary engines on coaxial quad for redundancy?

I’m designing a coaxial quad. It’s going to have 4 main high efficient motors with large props and 4 backup engines with small props that should only activate independently when the associated main engine’s throttle exceeds 90%. This is to create redundancy in case of an engine failure. In such a event I would also like to trigger a failsafe to land at the take off point.

Anyone know of a way I can code this in ardupilot to function the way I intend it to?