Adam Savage and Jamie Hyneman as Holmes and Watson!
This is the most clever idea I have heard!! Or, as someone else mentioned, they could have been working for the Home Office (without knowing who that really was, of course.) Then they show up in Storybrooke trying to figure out why they have lost all contact with the Home Office.
However, I must say my own personal favorites would be a couple of other classic Disney characters: Mary Poppins and The Absent Minded Professor. But I have no clue who could play either one.