Ditto zona's comments. First criteria is not stoker shoulders but pilot's derriere:
is width a potential problem and where the bars will be relative to the pilot's
rear. On a setup where the stoker bars are below the pilot's saddle and fairly
close to the saddle, you may find dummy brakes on a drop bar too close to the
pilot bottom, or not. Pilots with hip sizes in the 36" waist range will be potentially
problematic. If this is not a hurdle then you can fit what ever bar the stoker
finds comfortable. The drop position on drop bars may or may not be available
depending on how much proximity and flexibility are present.