dual recieve
Hi Well to get this out of the way if you are in the USA it is not legal, the radio guy would have to modify your radio out of the box, minor mods but do you really want to have your warranty voided etc?
I am not sure he knows that and if does not know that I would be very careful moving forward with him. If it is a radio guy with a business band radio business he is out of his mind putting the whole business at risk to do this. If he is doing it for others ( like a pubic servie agency) at some point he will have a problem and everyone who he has worked with will also have a problem. If he is a ham radio guy that is doing this part time at some point he may be caught and he should know better. If you live in a rual area chances are you and the radio guy will most likely not be caught but for the money saved you have to ask is it worth it?
I have a very unpleasant thoughts for hams that do this they know better and passed a test to prove they know better
SO a quick very cost effective alternative:
Purchase a legal radio with a priority scan or other type of scan functions, I am aware of radios like the VERTEX 3000/2200 etc that are very cost effective and allows you to do a priority scan for the business channel and when that is not active it will allow police and fire to come though. Also allow you to use a seperate scan for police and fire only or just the business channel so when you are off duty and want to get a way from the business you can.
Do not know if you need UHF and VHF or how many channels each group needs, I noted that you need both bands but there are some legal radios that allow dual band recieve etc. I have a VERTEX 1000 coupled with a VERTEX 3000 that does exactly what you are describing, legal on ham, public saftey, GMRS, business etc.
Main draw back, unlike the ham radios these radios need to be programmed the cables and software are not in-expensive however if you do not need to change frequencies then not an issue, the radios with a seperate control head are very small and easy to integrate into your vehicle.
One other things, LE gets very nervous and will look further if they see the radio in your car with a microphone attached to a radio with police traffic coming from its speaker. In other cases I am aware of this is how the people get caught. I am not a lawyer however one might think that LE has probable cause to look further if they see a radio like this. And if your work with LE they will resent this when they see it.
I am real careful to never leave PD or fire channels on my ham radio when out of the car, also it is very obvious that you cannot transmit on these channels and have showed LE on one occasion when they asked
PM for more information, there are some excellent prices on this type of equipment in the used and new market. I have this set up in my car for the same reason as yours, I would be happy to help you with the programming at no charge if you end up buying a radio that I have the cables and software for.