There really isn’t any point in doing that. 1) You should just use one of the two. Django officially supports Postgres. 2) You’d want to keep your users all jumbled in one database. If you have a foreign key to the username field, and the db can’t find a specific user because it’s in the other db, then what do you do then?
You wouldn’t need a REST api to do whatever your case is. Django supports using multiple database; just check their doc with a google search. It’s just a lot more business logic, and in my opinion, a lot more work and inefficiency.