architecture – Should 2 different MySql databases from 2 different applications be joined together if the 2 apps are merging into 1?

This is a question about software architecture, not databases.

I currently have 2 completely separate web applications, call them A and B for simplicity. They are on their own machines, with their own databases and web servers. My job is to integrate these 2 applications together (how I’m doing that from the front/backend is irrelevant here).

My scenario when it comes to the database is this: previously apps A and B were separate, but now B is being treated as a module, and will be used inside of A. To the user, it will seem like there is only one application. Behind the scenes though, is it best practice to keep the databases separate, or is it best for app A’s database to completely absorb the tables from app B’s database and just create one, very large database?

The data isn’t related between the two applications, so to me it makes sense to keep the 2 databases separate since B is acting as it’s own module still, regardless of whether it’s a part of app A or not. What do companies that have a large application do? Do they create a new database for every large component of their software or do they keep it all in one place?