How to work with 2 GIT projects in one project?

Question

git

Here's the question - what would be good GIT practice in such a situation?
The project on Yii2 (advanced), allocated the domain core with its namespace to a separate directory,
set up aliases, and made a separate repo on Gihab for this directory.
It turns out 2 GIT projects. I have never read in practice - is this normal?
What are the features and how to do it well?

Answer 1

https://git-scm.com/book/en/v2/%D0%98%D0%BD%D1%81%...