We use Kevlar bead 28mm Conti GP 4 Seasons on our tandem. We don't generally carry a spare tire onboard, although the one time we had a spare I needed one - go figure. These tires have proven very easy to mount and dismount; they can be mounted without tools and if I worked at it a bit I could dismount without tools.

I think there is anecdotal evidence that says Kevlar beads expand less at high temperatures something that's a bit more important for a tandem.