During the EA conference (which just ended a few minutes ago) the company revealed the brand new Unravel Two, which can now be played co-op.
You still have the option to play alone, basically, by playing both characters. While solo, you carry around the other Yarny until you reach a puzzle area that requires you to use the other character, simply by switching between the two. Your yarn is connected between each Yarny, so you have to interact with both to get through the game.
Unravel Two was developed by the same studio as the first Unravel, Swedish developer, Coldwood Interactive. Unravel was released in 2016, and in May 2016, EA announced they would extend their partnership with Coldwood.
Unravel Two was kind of expected to make an appearance this year at E3, but it was a big surprise to hear that the game was completely finished and available for purchase, today.