if you will regularly use both cars, i'd be getting two seats - having seen my SIL stranded at home when bro took the car with the car seat in it, not realising she needed to go out, i decided if we were in that situation there would be a seat in both cars. we have a 4wd and a ute, so no, we won't put a car seat in the ute, but my folks are buying a seat for their car in case i go out with mum or anything, and SIL has kept one of their seats just in case it's needed.

luckily the day SIL was left stranded, i was still living just around the corner, and had bought a car seat for my car, so i took her where she needed to go... sometimes you won't have that back up though!