Any chance you have a smaller granny gear laying about. That is pretty simple swap and can be a big difference.
If you could find a decent used 32-xx cassette and that granny you would know for certain.
Reason I mention it was your road tandem may need to be a donor of the cassette for a few miles of off road use.
I forget what we have on our off-road tandems but probably 24/32 and like TandemGeek mentioned, if we don't get tired, normally it becomes a traction problem. One section of a local trail has us climb straight towards the top. From riding the climb for many years and using this little wikki "eyeball it chart"
http://en.wikipedia.org/wiki/Grade_%28slope%29 it is around a 100+ feet at I believe 50% or slightly more. I'll ask some of the others that ride theirand get their thoughts on the grade. I'm likely overestimating it but hate seeing it come into view. My estimate is also not from riding it, but accomplishing many hours of trail work most of it cutting grass with a bushhog mower. It is a "B" to do and I fear breaking the chain every time. This trail also has many other climbs in the 10 and 20% range, though nothing is of a long distance. If you wonder how such a flat location can have these climbs, years ago, West Palm became home to the first MTB trail on a reclaimed landfill. It's not near as bad as it sounds. The landfill was allowed to do what it does for many years then covered with soil, years later Solid Waste Authority and the county converted the entire place into a huge park with ball fields in the flat areas, an RC airport, and two mtb trails, plus for this team, a place to ride the road tandem with a little bit of climbing.
PK