I have actually built a tandem bike with a jack shaft. It originally had a two stroke motor but am now working on puting a harbor freight engine on it. http://motorbicycling.com/f37/anothe...mpt-14533.html
. I never intended to have a rear passenger and my design would not allow one either. If you want a passenger then Id reccomend a sick bike parts or Thats dax shfit kit and putting a freewheel on both cranks. I'd be expensive but I'd work. Your best bet is to call sick bike parts and tell them what your trying to do. good luck.