suction cup type racks
I used a Seasucker Mini Bomber bike rack for a while. It worked great, including on multi-hour motorway trips, and I'd happily recommend it to anyone needing a bike rack for occasional use only.
Benefits:
- No need for crossbars on your car. Great if you either can't fit crossbars, or if you don't like the aesthetics of permanent crossbars (e.g., you have a sports car).
- Easy to lend to a friend or family member.
Downsides:
- Takes a bit of time to install properly, including cleaning the mounting surfaces. That's not a problem if you only use it occasionally. In my case, the only reason I swapped to crossbars and "regular" bike racks is that our occasional trips became frequent trips.
- You have to take the front wheel off and find a place to stash it in the car.
- The front mounts are typically for quick release bikes, you'll likely need to buy adapters if your bike uses thru axles (12mm or 15mm). They can be found for not much money though.
As for one two bike rack or two one bike racks: If I just had to carry one bike most of the time, I'd get two one bike racks. Otherwise I'd get one two bike rack.