The transaction is touching two different databases. So it makes sense that the two actions should be atomic and durable together using the one single transaction.
However, databases implement durability and atomicity using their own transaction log. Each transaction log takes care of its own database. So from another point of view, it makes sense that these are two separate transactions.
Which is it? Two transaction or one transaction?
Click through to read the article, and then check the comments for a cautionary tale around database mirroring and cross-database transactions.