We have two vehicles, but one is also in the fritz. My husband takes my car to work often if he will be driving around to appointments, and I can't drive his ca b/c it is a truck that wont fit cars seats (and I cant drive stick shift anyways!)
So, we do like you, he takes it on days only when I know I won't need it. But, we have had one car at a time, and I would take him to work on days I needed the car. But I always felt a little anxious at home without a vehicle, like what if there was an emergency, or those always seemed to be the days we got invited out on a last minute playdate or something. You could always do your plan, trade them in for a mini van, or trade in both your cars and get two used vehicles. You can get used cars that are in really great condition, for a pretty good price right now.