There are three keys to tandem satisfaction - frame, fork and wheels. For performance and durability, tandem wheels must be designed to carry two strong riders. Because the dished wheels found on many tandems tend not to survive nearly as long as dishless and good tandem replacement wheels cost about $500 each, it is prudent to get a tandem that has dishless wheels to begin with, if possible.
Tandem specific rims have 40-48 spoke holes, thicker walls to prevent spoke pull through and stronger joints to withstand doubled rider weight and higher tire pressures. Tandem hubs require stronger and beefier bearings, flanges, axles and bodies. Most crucial is that the ratchet mechanism withstand doubled drive torque - unfortunately, all single bike hubs come up short.
Because 26" wheels are smaller and stronger given the same number of spokes, many builders of all terrain tandems employ 32 or 36 spokes. This is probably not a good idea if you intend to take the beast off road.and don't want chronic wheel problems. A better choice is a 40 or more spoke wheel -- both front and back. Nearly half of a tandem's weight, including most of the captain who is often the heavier rider bears on a front wheel. Unlike a single bike, the front wheel on a tandem can't be lifted over potholes, rather it must endure the full brunt of all the bumps you encounter.
Symmetric or"dishless" wheels center the rim between the hub flanges. With equal spoke lengths, tensions and angles on both sides of the wheel, dishless wheels distribute the load equally instead of forcing half the spokes to carry a majority of the weight. To resist side loads, a tandem's wheels also need extra width between the wheel's centerline and the right hub flange.