I have in my head (which is a pretty warped place, TBH) a picture of a covered trailer, rather like those used to transport gliders. Would have to be a custom jobbie, but could get two tandems either side of a central plinth that holds the rack, and towing stuff.
Then - hitch it up to a Smart car
.
Sadly, I suspect it would tip up the car.
Having been transporting various tandems a lot in the last couple of years, we've firmly come to the conclusion that inside the vehicle, without any disassembly is the best option, so I'm occasionally browsing Van deals.
Outside the car has drawbacks; Noise, faff level, disassembly required on rear racks, reduced performance, stone chip damage, and in winter - potentially hours and hours of gritty salt-spray.
Inside with disassembly (say, a large estate car) also has issues; faff level, high risk of breakage or disturbing sensitive parts such as rear mech, handlebars poking you in the side of the head, etc.