According to this forum post, games with local multiplayer support will soon be getting online multiplayer support via Remote Play Together on Steam.
Remote Play Together is a new Steam feature that enables two or more players to enjoy local multiplayer games over the internet, together. We think this feature will be very valuable for customers and developers and are excited about the beta. We’ve provided an FAQ at the bottom of this message which we think addresses most questions and concerns. All local multiplayer, local co-op, and split-screen games will be automatically included in the Remote Play Together beta, which we plan to launch the week of October 21.
