auto_complete_for and more

Using the auto_complete_for method to fill a field is fine and dandy. The controller filters down the list and then you get your object. But what if you have a lot of the same elements in a field like last name? Once you use auto_complete_for you will have a picked a single object with that name. How can one then go on to a second step, and auto complete a second field based on the results of the first select.

Say I select last_name "Smith". There's 1000 rows that come back from the database. How can a second field, likely on first_name, be used to pare down those rows, preferably without going back to the database.