Keying relay.
Hello.
The normal solution is a keying relay, something that disconnects the scanner or other receiver from high RF levels.
There are keying relays that work on RF level alone, so except for the power connection and of course the RF connection there is little to do.
Most are now solid state, PIN diodes, just like an RF TR switch.
2 diodes back to back will work also but will add RF noise to your signal.
So, the best solution may be to simply herd into the junkbox.
The RF switch a nintendo uses works very well as a low power keying relay.
Power is derived from an RF preamp that drives a diode bridge.
So, RF level exceeds a given level and the preamp goes into saturation, driving the diode bridge that drives the PIN diodes.
This is what is inside just about every modern military radio made, EMP protection.
On military radios one must reset the radio once the event is over, if anyone is still alive to do this.
The one I got was military surplus that was intended for a multi-unit installation of the AN/VRC-12.
After that, with a firm understanding of how this works, I have been able to run several radios in vehicles with no trouble whatsoever.
And, if you are worried that 2 diodes are going to fail you need to reexamine your proposed install.
Even if you can keep the RF out of the coax, the RF levels are too high on the other antenna(s).
Remember, an antenna is intended to be away from other objects, including other antennas.
An antenna that close can easily affect the SWR of the transmitter.
Normally, a VHF 100 watt transmitter antenna mounted on the trunk lid will give no trouble to another antenna mounted on the roof, both distance and vertical separation come into play.