That will depend on the electronics inside. If it has twin recievers then you get 24/7 on both. Some of the high end RDS car stereos used to have 2 recievers - one would look for the next stongest signal, and then it would switch when the one you were listening to started to get worse than the next one it had found. Only really worked to any great effect on the Beeb national stations tho'........
Having said that, remebring the VHF is the prime safety thing, I would imagine the switch rate is quite high, and will switch to VHF as soon as it detects something.
I also like the idea of the whole lot in one box, but it also has the disadvantage that if / when it breaks down, you've lost the lot.