Ritterview put up a very useful post that illustrates the tire pressure problem for tandems, which is that tires and rims are only designed for so much circumferential pressure in the case of tires, and so much lateral pressure in the case of rims. If you look at the 143 lb./wheel value in the BQ chart, you'll see that line pretty much intersects the highest recommended inflation pressure for the very best tires of each width.
Here's Mavic's chart of maximum tire pressures for each tire width:
http://www.mavic.com/sites/default/f...echart_eng.pdf
From their chart:
23c = 138 lbs.
25c = 131
28c = 117
32c = 103
One can exceed these rim pressures, but they'll bow out sooner from brake track wear. I don't know of a 32c tire with a max pressure of over 100 lbs. Also, wide tires with high pressure ratings are usually slow, because they have heavy casings. Your C'dale probably came with 19mm wide rims. You shouldn't fit tires over 28c on such rims. If you want to go wider, put on 24mm rims, Velocity Dyads being popular for tandems.
However IMO, the what-tire-to-run problem is a separate problem from the why-does-our-tandem-feel-slow problem. Your bike will feel and get faster with more practice. It's the tiny differences in pedaling style between captain and stoker that have by far the largest effect. That's what makes your legs feel tired and the bike feel slow. What makes your arms and shoulders feel tired is not fit, it's that you're working too hard to control the bike. All that gets better with team practice. Our team has mostly given up riding our singles in favor of riding the tandem. This has made a huge difference in our speed and endurance as a team. Different teams will have different results, depending on how easily the team synchs up. We solve the effort difference between captain and stoker by both wearing coded HRMs so we can keep our HRs synched up, too. That makes a big difference over a long day, both in speed and emotional climate, as one team member doesn't get more tired than the other.