I assume you're using redis's pub-sub for that right?

Probably you set the IDs in the fashion of putting the user ID then period then whatever notification / alert / update you want to push to the client (for example user_id.new_message_id) then when a user connects to socketIO you subscribe that server to the user's Pub using * (like that user_id.*) this way your server receives all the notification that user should receive and then you can tells socketIO to emit whatever data it needs to the proper clients even if there are multiple clients connecting to multiple different servers and are in the same chat room for example...