According to the Steamworks developer site on Steam, Valve is working on a feature that will automatically add online multiplayer support to local-only multiplayer games. Remote Play Together is expected to launch in beta during the week of October 21. Details like network speed requirements are still unknown at this point.
https://www.notebookcheck.net/Steam-s-Remote-Play-Together-adds-online-support-for-local-multiplayer-games.438275.0.html