It will depend on the location. You will not be the first person to do this and won't be the last.

For a regular reservation in most cases there is no charge to no show. If there is a fee it will be disclosed during the reservation process. Tere will be a fee if it was a prepaid, based on when you cancel.

Of course, its not helping the tight on cars situation when people reserve multiple cars, but that is a different story.

They don't know if you need one or both cars, so they will probably prepare two assuming an additional driver would be driving the other one. Which one you get and which contract they toss out is anybody's guess.