I'm looking for 25mm tires and read a review on these tires on Road CC:
"The rubber doesn't wear away particularly fast, it's just that there's not much of it to start with, so if you're after longevity, these ain't the tyres for you."
That killed any chance as I read tandems do wear through tyres (or tires) faster.
"I've always felt that the first time I would lay the bike down and cause some hurt might very well be the last time I get to ride the tandem, and I want to keep riding the tandem!"
+1. My thoughts exactly.
Indeed the Ultremo ZX will wear faster but you get a much plusher ride on them. 4-seasons and Gatorskins from Conti will give you better mileage and the same with Ultremo DD and Durano from Schwalbe. Schwalbe tend to run a bit narrower than advertised but in some cases they are lighter than their Conti counterparts (Duranos are lighter than Gatorskins) - Great tires.