We test rode a Fandango 29'er iast spring at the AORTA (Appalachian OffRoad Tandem Adventure). The captain is an experienced mountain biker (still on 26" inch wheels) and I had no mountain bike experience. The riding was fairly technical for newbies and we did have a fall each day. Our biggest challenge was the much higher center of gravity.

We finally bought a used 26" wheel Ventana El Conquistador de Montanas a few moths ago. We liked the lower center of gravity (even though many people are going to the 29ers). So far, we've been very happy with it. We've ridden part of the trails that make up the Iceman Cometh race outside Traverse City, MI and joined the FART 2012 group in the Allegheny area of New York (they've been getting together annually for 19 years - we learned a lot from them that weekend).

I think the learning curve will be shorter if you ride with other experienced mountain bike tandem teams. We learned a lot at AORTA and FART. Also, if you are uncertain I'd recommend patience to find a used mtb tandem. We felt that if we didn't enjoy it, we'd be able to resell the bike for near what we paid for it.

2010 Calfee Tetra road tandem
2005 Co-Motion Primera Co-Pilot travel tandem
2002 Ventana El Conquistador de Montanas full suspension
.and Schwinn Twin cruiser tandem