We also have 2 car seats as it's so much easier than having to shift it all the time.
The days I work we go to work together in DH's company car, the days I don't work I use my car and on weekends we use DH's car again so we'd be constantly changing every day otherwise!
I guess it depends on whether you have a primary car or you use both cars equally?
Hope this helps!

ETA - my mum has DS while I'm at work and also has one in her car so I guess we have 3 lol