TandemGeek pretty much summarized it fir you. The MTBR tandem section has some great information. Alex at mtbtandems.com is the best source of information for purchasing a MTB tandem.
Here' a link to a few photos of off road tandems. FWIW, we started on a road tandem and a few years ago decided to try off road tandem riding, it did not take us long to become hooked. Now we rarely ride the road bike...
https://picasaweb.google.com/dan.orl...eat=directlink