I'd have thought that one rider regularly jumping over rocks and stuff puts a lot more strain on the wheels than two riders hitting the occasional pot hole.
My guess is that back in the good ol' days, it wasn't worth making a new rim profile just for a limited number of tandems, so the easiest way to make a stronger wheel was to put more holes in a normal road rim and hub.
Now that manufacturing processes have improved and got cheaper, and there are more DH bikes around than there ever were tandems, it's easier to make stronger rims and hubs.
Any truth in that ?
I'm curious because I'm building up another tandem and wondering what hubs, rims and tyres to use. There's far more second hand MTB than tandem stuff out there.