I'm recreational / fitness-oriented and have 2 road bikes.
Before I had 2 road bikes, I was stockpiling spare parts to try to minimize downtime whenever the bike had some significant issue. I had extra derailers, wheels, and pretty much anything I thought might need to be replaced after a crash or some wear and tear.

But yea my current, more conventional approach is just to have 2 bikes. I like it better. And in the last 9 months I've had a significant gap to cover with just one bike as I was upgrading the other (new frame even).

But when both rides are in good shape, I'm set up with some other intentional distinctions: a hill/wind oriented bike; and a speed/flat oriented bike. There is also one of the two which is preferred for leaving locked outside for a few minutes when I need to take one for an errand (rarely!). Think of your use cases like this when you're planning and you should end up with some nice ways to get complementary functions from your fleet.