I run a 30/28 low gear on my tandem. I've done some really long, in the 20+/- mile range, 2%-6% climbs. The lowest gear on the tandem isn't such a big deal there because we're not in it much. I've climbed 10 miles with an average grade of 14% and I've done some much shorter five mile climbs that have had pitches up into the low 20% range. We've been able to get up them pretty handily with the existing gearing.

I like the spacing on the 11-28. I'd be bothered with the spacing on an 11-36. The jumps are too big for me but gearing is very individual and you should go with what works well for you. As you guys get stronger on the tandem (and individually) you may find that you don't need such low gears.

As far as blowing up goes, that can be a big problem on long distance events (I'm sure I don't have to tell you or Rowan that ). Don't know if you use a HR monitor but that might be a useful tool to help keep you out of the red zone. If you know your body real well you can get buy without one (I have/do). If you need to, you may just have to stop and take a brake on really long or steep climbs to get your heart rate back down.