100psi seems a little low for a tandem, but at least it wasn't over-inflation that did it. I run 115 on the Conti "Ultra-Sport" 700x28 that came with our CoMo. Still rolling, after 2k miles. Front looks new, back is starting to get square edges. We do not carry a spare, don't even own one at this point, but probably should pick up at least one. I don't know about rotating; that just seems like work. It isn't like we are racing in the rain or anything. I would definitely say sidewall blowouts on tires are rare. Unexplained ones even more so. You have now had 2, and that seems like an awful lot for something so rare.