How would you possibly guard another frequency?
Exactly the same way the marine VHF radios do it.
They have a priority scan system that monitors both CH:16 and another.
If 16 is busy it stops and you hear it.
If the alternative channel is busy it stops and you hear it but it also continues to sample 16 (so fast you're not really aware) and if something come up on 16 it automatically switched there.
Then there are marine VHF that can 'triple watch' with 3 channels.
All with just one radio.
Been doing it for donkeys years.