When are two rudders necessary?

A single canting rudder lets the blade stay perpendicular and hasn't the drag of a second rudderhead, even if the blade is up.

 

now that would be cool, got any photos?

 

There are some pics & info on canting rudders as PAR mentions in the Dashew's "Offshore Cruising Encyclopedia" vol II. I think that said appendage, along with a LOT of other neat innovations is on a semi-prototype Hunter/Custom cruiser.
For $50, the books is DEFINITELY worth adding to one's library. It's got likely close to half a century of professional grade boatbuilding, design, & maint. knowledge in it.

 

I'm not qualified to theorise.
At one time years ago, I owned an E-scow.
Very fast.
Twin tiny balanced aluminum rudders .....and twin aluminum bilge boards.
if I remember right, the boat was 8 feet wide, 26 ft long, and 18 inch depth of hull.
A BIG sailboard.

 

Here's a start, info & image wise on canting rudders http://www.bing.com/search?q=Lars Bergstrom Canting Rudder&qs=ds&form=QBRE

As to Yobarnacle's comments about E-scow's, I can STRONGLY second them. When the breeze is up those things are Rockets. We took a buddy's out on San Diego bay for a race when it was blowing low 20's, gusting to 30, & spanked everything out there. Including all other boats from "The Wizard of Zenda" (Buddy Melges), as in 24's (it was a while back).
They're definitely an E-ticket ride.