Let's say a server gets 10,000 concurrent connections (via socket.io). That's a lot, and if it can't handle any more, I need to spin up another server.
How can I sync the two servers together with their socket.io?
You can try to use for example cluster module and distribute the load to multiple cores (in case you have a multi-core CPU). In case this is not enough you can try to use reverse proxy for distributing requests across multiple servers and redis as a central session data store (if it's possible for your scenario).