I wouldn't own two identical-purpose bikes unless you are real, real sure you won't need a different bike.
1. Road Bike
2. MTB
3. TT
4. Cyclocross
5. Commuter or hybrid
6. Vintage road
This even solves the issue of noticing at the last minute that you have a flat. TT bike can go on a road group ride. Fitness ride you can take a cross or MTB. Etc.