Multiple projects with almost same database structure best practice

I’m seeking advice for building a database.
currently we have a database for a project which is mongodb, we are going to have another 2 systems almost with same domain goal just different data but almost same models with slight differences just like different types added for example.

the need is the 3 systems will need to communicate data with each other, for example creating a user on a system should reflect on other 2 systems so whenever you are going to search for that user from any system you will find it, and also insertion should make sure that data is already there while inserting from system 1 a record that already exists in system 2.

my recommendation is having multi tenancy, shared core , that has the needed data across all 3 systems, but despite having same structure per database i think it is better to separate them each system has its own data ( except for the users here ) .

suggestions for designing databases and how systems will communicate ?

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