As an addendum to the twin tail rotors post, the Apache, if you look closely, has two separate two-blade rotors on the same hub, offset by something other than 90%, with separate pitch links.
When I noticed this on an Apache on static display at Middle Wallop earlier this year the nice man in a green suit said it was for survivability, and each pair of blades has sufficient authority to control the machine should the other pair be lost. Some interesting design criteria there.
I've tried to find static piccies of the t/r, and can't - all the piccies I've seen are of the beast in flight . . .