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.