Your current 26" wheeld tandem can be outfitted with a good suspension fork. That takes care of the front end of the tandem but leaves the stoker's without suspension! A good shock absorbing seatpost would help her some, but is not ideal. A specifically designed off road/moutain bike tandem with full suspension front and rear would do it for hopping curbs. . . however, hopping curbs is only a mnute part of tandem riding. A fully suspended off road/mountain bike is not the best thing to be riding on the road.
There are many 700c tandems, quite often referred to as 'road tandems.' While a 26" wheeled tandem is ideal for light off road/trail use without suspension, it can also be utilized on the road. 700c wheeled tandems are not to well suited for non-road or dirt road rides, although we have done it, but it's a bit precarious.
In the US there are more 700c tandems than 26" wheeled tandems.
BTW there is no such thing as 'a stupid question!'
Pedal on TWOgether!
Rudy and Kay/zonatandem