Sharing things between CI4 instances

For a number of reasons, I'm toying with a "split app setup". There are basically three separate and isolated CI4 "instances" or "installations" serving the same platform. One is used for signing up/registering, one is used for logging in, and one is used once you're actually logged in.

There are obviously things these "instances" have in common, code being the most obvious item, and in some cases also data from databases.

I would like to keep them separated, but allow for "code sharing". Is there an official/recommended way of doing this, without having verbatim copies of each class, etc. in each instance of the "app"?

-joho