What is better in postgresql, 2 databases smaller or 1 large?

Question

PostgreSQL

There is one ~1TB database on MySQL
There is another database on PostgreSQL, it is new, but the content in it will be about the same terabyte in a year,
the database with mysql, we want to transfer it to postgres,
but the question is: add information from it to the same database that have on postgres, or create a new one?
between these two databases, there are several common tables,
I thought in postgres to create new schemas for the database with mysql, but I'm afraid that when the sizes grow, there may be problems
on the disk, there is space, SSD, everything is as it should ...