There are people who specialize in fixing bike fit problems, e.g. Andy Pruitt, but whether there is anyone good locally I'm not sure.

If my heart rate starts to climb too high I try to force myself to slow down until it stabilizes. Of course, there comes a point where you risk falling over on a tandem! Standing up typically raises your heart rate, so it's not a solution, just temporary relief. I always go to a higher gear before I stand and then drop back to the lower gear when sitting. You're right that you should be able handle all but the steepest hills with the 24-30. We're still running the 24-32 and I'm always (pleasantly) surprised that we don't run out of gears as fast as I do on my single road bike, which has 28-27 as the lowest. We go slower, of course, as my wife only rides the tandem and isn't a strong rider, but we can handle 10-12% and still be spinning at a decent cadence.

I think you're doing pretty darn well. As I recall, we couldn't keep up with you on the Sonoma Tandem Rally! The endurance will build steadily over a couple of seasons if my experience is anything to go by. I ride my first century in Sep 2007 and have completed 8 double centuries this year. My wife thinks I'm crazy, which is probably an accurate assessment.