I have multiple teachers who teach the same course. How can they have separate Moodles and still share question banks?
If they're all teachers in the same Moodle course, they should be able to share the course's question bank already; just make sure that the questions are added to categories within the course rather than only within the quizzes.
If they're using separate courses within the same category, you can create a new role and assign it to them at the category level so that they can add and use questions within the course category context - see the Moodle docs for information on setting that up.