You can't. GQL is not SQL.
You will need to to do one select, then iterate and fetch/filter or do two selects and merge in code.
Alternately add a flag in the Car object that tells you that the Owner is a User, then you don't need to
If you are not trying to write code, but just explore then the remote_api shell is a place to try these things. Though this is much easier in python ;-)