Sounds abit rediculous and out there. But has anyone every thought of building a double necked shamisen? Japanese steve vai perhaps?haha
There is one working double neck shamisen in the movie “Overdrive”, also called “Three stringed samurai”
Yeah. Unfortunately it gets destroyed at the end of the movie!
(oops! Did I just give away an important plot point?)
Sorry movie fans! but still Overdrive is a must-see movie!
As Kyle has pointed out in his post its a bit weird at times and tends to wander off into cheesy Japanese pop culture drama (silly immature personal problems of the characters that noone really cares about) but it makes up for it with some of the GREATEST Shamisen battles and some of the most awesome Shamisen playing footage ever captured on film. Masahiro Nittas breathtaking Tsugaru Shamisen skills explode onto the screen in some of the best scenes. Yoshida Brothers also make a cameo as well and Kinoshita Shinichi displays his Absolutely mind boggling Tsugaru technique for a full 2-3 minutes in one scene where he visits a master Shamisen builder!
My favorite scene though is when Gen (the hero) stumbles upon the Legendary Shamisen playing demon of the forrest (complete with Oni-Devil make up, Samurai dress and a Bachi that was forged in the fires of Hell) and they battle Dragon Ball Z style with trees and branches tossing around in the wind in a wild frenzy around them as a result of their intense Shamisen playing!
So about the double neck Shamisen (Steve Vai style) Yes. Its been done! Its out there.
(unless the one in the movie was the only one ever made?)
Geez, Kevin! Talk about a spoiler alert!! 
Sorry Everyone! Didnt mean to give so much of it away. Was shooting for more of a review than a spoiler! Oh well... If you do plan to see this movie be sure NOT to read anything I have written here or on Kyles post called OVERDRIVE.
Damage already done 
Although just the first part… Maybe you should edit your post Kevin? 
Hahahahahahaha!!! Nice save, Kevin. ;-D
Yeeeaah…to late but boy do i want to watch it now! But what would the second neck be? Bass,6string Shamisen? totally different neck style? Jiuta+Tsugaru. sing while playing on the jiuta neck and solo with the Tsugaru neck?
Deshaun,
OOOOOOOOH, Its so tempting to tell you but seeing as I have already gotten in trouble for giving so much of the movie away... I guess Ill have to just resort to the age old generic reponse…" You just gotta see it for yourself!"
(I will give you a hint though… One of your guesses is Correct!)