How to route traffic for a collaborative editing system in Node.js

I have a Node.js application that uses sockets.io to allow multiple users to edit the same document at the same time (a little like Google Docs).

I want to scale this out horizontally, but I'm stuck trying to figure out how to ensure all the users editing a specific document end up connected to the same Node.js instance.

For example, assume there are 2 documents (doc-A and doc-B), and two servers (server-1 and server-2). Someone connects and starts editing doc-A on server-1. Another user connects, also wanting to edit doc-A. They must be sent to server-1 to ensure the document remains consistent. Finally, a third user connects wanting to edit doc-B. We could send them to either server as no one is editing that document at the moment.

I'm sure every collaborative editing experience must solve this problem, but I can't figure out how to do it. Can anyone help?