Design suggestion of multiple databases vs multiple schemas within the database

I am building a web app with Postgres, that also uses Drupal with Postgres. I am new to all these frameworks.

There is some data that I'll need to cross-reference between the two databases.

Can I do a cross-schema/catalog join? Or is a cross-database join better?
Are there any gotchas for the cross-schema/cross-database joins that I need to be aware of? Such as when using Shared hosting?

Any suggestions/comments appreciated.

Thanks