I built a touring tandem up in 1987 using the then fairly new 26" MTB rims. I could only find these rims in a max of 36h, and was a bit dubious about this as our previous touring tandem had used 48 spoke wheels front and rear. However, seeing that was all I could find, I built the wheels 36h both front and rear (140mm spacing on the rear, Bullseye tandem hubs), and 33 years later we have yet to break a spoke or have a wheel go out of true, though I've needed to replace rims a couple of times due to sidewall wear from braking. The all-up weight of us plus tandem plus camping load is about 400lbs, and we've had it over some pretty rough unsealed roads in the outback.
On our racing tandem, where it's only ever ridden without luggage (Co-Motion Robusta) I used 40h rear 32h front with Phil Wood hubs and Velocity Deep-V rims, and they've similarly remained dead true.
My Tandem team weight with Pregnant wife was close to 400lbs We were also pulling a custom two wheeled cart that weighed around 50lbs empty. The only rims with 40/48 drillings I new about at the time were made by Sun.