All the 12 string basses I've seen were 
  doubled upper octaves. That is one piece of gear I must own someday. The sound 
  is positively massive. Best exponents I've heard of the instrument are Doug 
  from King's X, Tom from Cheap Trick and the guy from Big Wreck (the tone on 
  "That Song" is awesome). I believe Hamer 12 string basses are the most common 
  I've seen around 
   
  ** yeah most are like that. the late allen woody had 
  an alembic 18-string bass. 6x3 . . . there are a few people that are 
  making the x2 rather than the x3 versions: warwick has, fodera, 
  warrior.
   
  stig