Hosting two logically separate apps on one node.js server?

Currently I have two apps each with their own node.js server, that means there is a lot of duplication, the node_modules are similar, the configuration is similar.

One app is served on the main domain, and the other is served on a sub domain.

The biggest thing is the sessions are separate, so I can't log into one and be logged into both.

How do I create one server to work for app.example.com and example.com so that they share common node_modules and configuration, but I can tailor specific needs for each app in that server?