Single car + occasional rental can definitely be a solid strategy. It's worked pretty well for us for almost two years. Renting a car sometimes "feels" expensive, but really it's just showing you all the actual costs of driving at once.
Recent example: last week GF rented a car for six days (Mon night - Sunday night). Cost was $145, plus 12 miles (6 miles each way, about 10 minutes) of driving in our car to the rental place to drop her off to pick up the car (12*$0.25=$4) and an Uber for her to come home after dropping it off ($12). She drove the car 652 miles. $161 divided by 652 miles is $0.247/mile. The car got 34MPG while she drove it (Corolla) and she paid $2.70/gal for gas (7.9cents/mi).
That works out to $0.326/mile for the rental car. According to my spreadsheet, our car costs us about $0.25/mile all-in (gas, maintenance, repairs, insurance, registration, etc). So the premium to have the rental car for those 652 miles was $.326-.25 = $0.076, $0.076*652 = $49.55.
Call it fifty bucks. That's about what it would cost to insure a second car for a month, not to mention opportunity cost of tying up more capital in a vehicle or dealing logistically with a second car lying around. We on average do this dance less than once a month (and usually at a similar cost), so it pencils out as cheaper.
Run your own actual numbers, but I was surprised how well it can work out for us.