I think PMK and Tandemgeek covered it already, but here's my .02 cents.
Similiar to your situation, my wife and I ride our tandem 2-3k miles a year. On one of our tandem trips, we did a little off road riding and my wife thought it would be fun to get a mountain tandem. It did not take a lot of persuading for me - a few calls and a few more emails with Alex at mtbtandems.com and we had our new Ventana ECDM on its way. I highly reccomend Alex, he is a wealth of information and great to work with!
Our Ventana is the most ridden bike in the garage over the past 15 months. We REALLY like riding that bike. I'll reiterate what Tandemgeek said, where you can ride a mountain tandem is mostly a function of your fitness (tight switchbacks seem to be the biggest limiting factor, but you can walk them). Living at the base of the Rockies here in Golden, Colorado, the majority of our rides entail a lot of climbing. We can definitley ride grades in excess of 20%, but not sustained for miles... also depends on the trail, really loose stuff is can be challenging like on your single bike.
+1 on the DoubleForte group on Yahoo. Good group.
+1 on mtbr.com tandem section. Not a lot going on there lately, so definitely join and ask questions! I'm "DS2199" on mtbr.com - PM me here or there if you have any other questions.
Like most around here, I love to talk tandems and lately favoring the off-road ones!
Dan
Dan