Slime Rancher was designed to be a single-player experience and adding in multiplayer functionality would require a massive overhaul in how we originally built the game, assuming it is even possible! (The physics simulation in Slime Rancher is unlike any other game on the market!)
Most of Slime Rancher's development was done by two people, so a lot of decisions were made early on to allow two developers to make a game in a realistic time-frame. One of those decisions was multiplayer vs single-player functionality.
Multiplayer is inherently more complicated and time-consuming to develop. The hundreds of physics-enabled slimes bouncing around on screen needing to be in sync across two or more clients is a fantastically difficult problem to solve. As a result, we decided to focus on solid single player experience.
For these reasons, we would like to officially state that Slime Rancher is being developed and will continue to be developed as a single-player game and there are no plans for multiplayer at this time.
Thank you again for all your support and enthusiasm. We know that your multiplayer requests come from a place of love and we're just as excited as you are to see what comes to the Far, Far Range next!