Share 'Strings on the back. One neck, or two back to back?'

Something Ted said made me think it might be possible to put 10 or 12 strings on the back of an existing 10 string instrument's neck. I thought I absolutely had to have two necks back to back and with trusses. Maybe that's ideal but not absolutely required. What do you think? Can I get away with only one neck? What would be the best way of doing that?