Good afternoon how can i joined 2 tablea from different database like database A user to database B user i want t combine them

Question

When a user B login chat section he is able to use database A user content email and password then login instead creating new tanle user in B to avoid user making 2 accoumts

Answer 1

different table structure in different database ?
or
same table structure in different database ?

Answer 2

Different database but same table

Answer 3

Like database A and B have same common on users