What about a hand-built 36 spoke wheel?
It looks like I could have a pretty beefy mountain bike wheel built up for a lot less than the cost of these custom-spaced higher-spoke-count tandem wheels.
What I'm thinking (although I'm open to suggestions)...
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Sun Rhyno Lite rim (36 hole)
◘ DT Swiss straight gauge spokes (14g)
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Shimano XT 135mm 36 hole rear hub.