A couple of things that need to be considered. Usually these type units are designed for VHF to base stations, and UHF to the portables.... Crossband operations, all licensed under one System License under Part 90. Hams can do the same thing using their Repeater Subbands. FRS does NOT ALLOW repeater usage. GMRS however does allow Repeaters, but requires specific System Licensing. You can NOT mix Ham operations, with any other Radio Services, in the USA. Motorola made the best setups for Mobile Repeaters. They were very expensive, and we're setup on a very limited number of frequencies for each band. Usually 1 or 2 on the VHF side, and a single Pair on the UHF side. Usually they were Manual Channel selection, that depended on the area of Operation, with a fixed pair to the portables. Problems occurred when more than one mobile Repeater was operating in any one location. Things got messy with multiple VHF transmitters repeating back to a Base, triggered by the same portable. Some used specific PL Codes on the UHF side to alleviate this issue, but that meant that the Portables had to be assigned with the vehicle, and NOT the user.
There are a few Crossband Dual Band Ham Radios that can do this, if properly configured.