Among the finest to understand whether it's safe to possess two databases which are relational to each other, instead of the tables within the database being relational to each other? (don't misunderstand me the tables it's still relational)
The particular relations are often enforced through the application that's while using database. If you possess a relation that goes from database A to database B, you'll be able to just implement it like this inside your application.
Obviously by getting two different databases, you cannot obtain the internal support for relations you typically achieve with a home database system. So for instance you cannot specify foreign secrets and add rules according to them.
You could do this it but it is most likely not recommended. To help keep relational integrity from a table in a single database and tables in another you should apply certain kind of distributed transaction which provides coverage for both databases. This is much more complicated than the usual transaction spanning just one DB.
That's likely to be a headache if you are planning to make use of these databases by any means in which a DB product is helpful whatsoever. Transactions, referential integrity, etc. are just possible in multiple DBs should you control them from a 3rd party controller. For those who have any choice, it's most likely not worth keeping them apart.
It is dependent that which you mean by "safe." In case your tables are InnoDB, that you can do transactions across databases within the same server. Foreign key constraints apparently also work.
Enforcing either of those across several server is, In my opinion, difficult.