Does anyone know where I can get plans for the double hand chopper?

Are you describing the Double hole wrist chopper originally put out by Delben years ago?
There have been several people make a version but I don't recall seeing any plans, but a friend of mine said they may have been some from Owen's. You could try them.
I had one I purchased from Fairchild a couple of years ago. The apparatus design was really pretty simple, but the craftsmanship is what make the trick work.
PM me if you want specific details.