We only have one set of car seats and 2 cars. TBH i wondered this when pg too. But over time we have stopped changing car seats realising its easier just to use my car as the family outing car. If DH ever goes out alone he takes his. If we want to take one child each then we have to take one out and move it but this rarely happens as if he takes DD1 out i tend to stay home with DD2 to catch up on house work etc and then he just takes my car anyway. I do have a 2nd set of car seats that my sister gave me when her girls out grew them but i leave them in my PIL's car as they often baby sit so for me thats easier. I dont think you NEED 2 but its an individual thing iykwim.
Hoody