If there was only two vehicles, and both used two radios (1 FRS handheld & 1 scanner with an external antenna) you probably could. Vehicle A could be transmitting on ch 8 and listening on ch 14. Vehicle B could be transmitting on ch 14, and listening on ch 8.
I don't believe this would violate any rule sections.
If you can find them, the old mobile FRS radios from Radio Shack should give you some decent range. They put the transceiver in the magmount, and attached the 1/4 wave antenna on top of that, and the cable led down to the controls in the speaker-mic. The cable also split off to plug into your 12 volt outlet. (The thickness of the cable requires you to route it through a slightly open window, and that can introduce objectional wind noise
at highway speeds, not to mention rain/snow/heat/cold depending on the time of the year, and current weather conditions. I've used one in a pickup for a couple of AZ-to-MT and back roadtrips in '02 and '04, routing the cable through the split sliding rear window. If any rain water traveled down the cable, it either got wicked off onto the top of the bench seat, or continued down the cable behind the seat and dripped onto the floor under the seat. I wasn't worried about either. I was running 3 cables: the mobile FRS, a CB magmount for a handheld CB, and a 2m/70cm magmount for a TM-D700A. There wasn't much activity on any of them.)
John
Peoria, AZ