The Elder Scrolls Skyrim was released back in 2011, but thanks to the modding community the game has new content and remains relevant four years on from release. The modding community on PC have provided all sorts of additions to the game, from small texture packs to entire quest lines. A new mod currently being developed will allow players to invite their friends and explore the vast open world cooperatively.
The mod’s name is Tamriel Online and has been created by reddit user Siegfre. The mod is still in its infancy, not yet supporting combat, but it serves as am example of what can be done, and what Skyrim would be like as a two player game. Siegfre is currently asking for people to help with testing the mod, finding exactly what needs to be fixed with it. Already many problems have been solved including save issues and situations that caused the game to crash.
If you are interested in playing the mod, the instructions for downloading and playing it are here. Keep in mind that the mod is still new and as such will most likely not run smoothly. The mod at present is really a proof of concept. There is a video below which provides an overview of the mod in its current state.