Only the host needs to own the game as remote players stream the game from the host and play as if they are sitting right next to the host, like in the olden co-op game days.
We tested Road Redemption and Super Blood Hockey which worked great and the remote party did not complain about any lag. Keep in mind the remote players I tested with had a 100mb and 20mb fiber connection respectively.
To get remote play together working, launch the game, press alt + tab, go to friends and click on the drop down next to the friend you’d like to invite. Then click on remote play together. The remote player will get the invite, accept it and connect to the host’s session.
Steam currently has a sale running for games that support co-op and there are quite a lot of fun titles to choose from.