Today, Steam added a new feature into beta that allows players to join local co-op and split-screen games online.
According to Steam, the feature can connect up to four (or more in ideal conditions) players together, with only the host needing to own the game. Meanwhile, the others connect via Steam Remote Play streaming.
The feature is available by joining Steam Beta and launching a local multiplayer game. You then click ‘Remote Play Together’ from your friends list, and play together. Their controllers will act as if they are plugged into your PC, and video, audio, input and voice will be shared between players.
Steam Remote Play beta supports PC, Mac, and Linux.