If I were building a new tandem today I would look at SRAM 10 speed 11-32 for the cassette and use a double rather than a triple and go with a 36/52. That combination gives me the same top gear as we have today and a similar low gear.
We currently have 30/39/52 with a 12-27 cassette. Top is 114 inches and low is 29.3. With the 36/52 and the 12-32, top is still 114 and low is 29.7. Probably low enough, we climbed an 8% grade yesterday in the 30/27 with no problem, that is probably as steep as we will climb. For us that would eliminate the need for a triple. And the ratios are not that bad. An 11 speed might be another consideration to keep the ratios a bit closer.
Wayne