Steam is readying a feature called “Remote Play Together” that allows any game’s local multiplayer mode(s) to be played online. By leveraging the service’s current in-home streaming technology, up to four users can play together at 1080p 60 FPS.
All games with local co-op will support Remote Play Together by default, and nobody aside from the host will need to own a copy of the game. The feature will reportedly be available to test beginning October 21 when it enters beta.
Using the Remote Play Together Beta, a player can simply launch any game with support for local multiplayer, local co-op or shared / split-screen features and then via the Steam Overlay, invite a Friend to join their game for some multiplayer fun. The invitation is just like handing a second controller to a friend.