The meaning of the term 'database' varies with each major warehouse manager. Hence, the answer to "can a dbt project connect to more than one database?" depends on the warehouse used in your tech stack.
- dbt projects connecting to warehouses like Snowflake or Bigquery—these empower one set of credentials to draw from all datasets or 'projects' available to an account—are sometimes said to connect to more than one database.
- dbt projects connecting to warehouses like Redshift and Postgres—these tie one set of credentials to one database—are said to connect to one database only.
Sidestep the 'one database problem' by relying on ELT thinking (i.e. extract -> load -> transform). Remember, dbt is not a loader--with few exceptions, it doesn't move data from sources to a warehouse. dbt is a transformer. It enters the picture after extractors and loaders have funneled sources into a warehouse. It moves and combines data inside the warehouse itself.
Hence, instead of thinking "how do I connect my dbt project to two databases", ask "what loader services will best prepare our warehouse for dbt transformations."
For more on the modern 'ELT-powered' data stack, see the "dbt and the modern BI stack" section of this dbt blog post.