Contemporary databases do not allow two owners access the same record at the same time. This is how banks maintain balances of their clients or account-holders. In contrast to the set practice, Google docs allow both the parties to access the same document at the same time.
Moreover, it also allows to view a single version of the document to both of them simultaneously. Just like a shared ledger, the Google Docs also acts as a shared document. The distributed part only becomes relevant when the sharing involves multiple users.