Have you looked at the Pedaling for Parkinson's thread? There is a trove of information about the effects of high cadence cycling on PD symptoms. Briefly, it doesn't matter how far you ride or what kind of bike you ride. In order to have the benefits of cycling on PD, you ride for 1 hour 3 times per week. 10 minutes warm up, 40 minutes at a cadence of 80-90 rpm and 10 minutes cool down. (Note we are talking cadence here, not speed. Lower resistance until you can keep up the cadence.) You do not need a tandem. I use a road bike and have done that since early 2009. In the winter I put the bike on a trainer in the basement. I typically ride 4-6 days each week and ride the full hour at the higher cadence.. closer to 80 rpm than 90. Following this protocol I have been able to ride across Iowa five times, climb Mt. Kilimanjaro, climb to Annapurna BaseCamp in Nepal and climb the Inca Trail to Machu Picchu among other things. I'm currently at Whistler in British Columbia, skiing, not as good as I used to, but still having fun. My doctor is astonished that I am still at Level I on the UPDRS. So am I. I'm 70 years old, BTW and was diagnosed in 2008.