Multiple receivers switching

I’ve read manual and it says that it will switch only if there will be no pulses… FS_THR_VALUE will not work…
I have old-style receiver Arkbird 433mHz connected to rcin via cPPM and ExpressLRS receiver connected to telemetry port.

Is there any way to monitor FS_THR_VALUE value to monitor low values and switch to secondary rc link?

If you have an ExpressLRS reciever, why have an old Arkbird?
433MHz wont be long range or anything useful, its a noisy band of radio spectrum for everyones garage door openers and toys.

I need a backup rc channel as 900mhz can be jammed

Or can I theoretically switch rc input by lua script?