Contrary to modern opinion, tandem teams using well-designed 700c wheels can carry expedition loads over graded dirt roads without resorting to a trailer. Before it was paved at least three teams on Santana tandems survived 1500+ miles of dirt to conquer the Alaskan Highway. These teams used the 27-inch equiv of today's 700x32 tires. Because roadside amenities on the unpaved "Alcan" were rare (no AAA motels or campgrounds, and a sparsely-supplied store only every 80 or so miles) these teams not only packed a tent, clothes and sleeping bags, their overstuffed panniers and rack-top duffles also contained food, stove, fuel and even water. Not one of these teams used a trailer, and their standard wheels survived unscathed. Today's spokes, rims and tires are stronger.
The biggest wheel strength variable is not spoke-count, but rather hub design. The 140mm Phil hubs found on early Santana tandems and the modern 160mm cassette hubs used on most post-'92 models allow totally-dishless wheels PLUS a full 60mm of flange separation. Other hubs (including 7speed 140mm hubs used through the '80s and and 8/9 speed 145mm hubs still used by many builders) cannot provide this degree of wheel strength; no matter how heavy the rim or numerous the spokes). Still, if your combined rider weight exceeds 350 pounds, or you will use the tandem for extended unsupported tours, 48 spokes are a prudent upgrade (that adds 91 grams or a bit more than 3 ounces).
(And no, the report about a tandem with 48-spoke wheels being unstable in side winds is silly.)
The undeniable advantage of a 26-inch wheel is what I call "bash- strength"---the ability to slam through ruts at high speed. The unpaved roads encountered on most tours are graded well enough to be travelled in a family sedan. On these sections a loaded 700c tandem with 32-35mm tires will survive just fine. If you want to use jeep trails (or worse) 26-inch wheels are the only ones with adequate bash-strength.
Surprisingly, because of their wheelbase and weight, tandems can get by quite nicely with tires that would be too smooth or narrow for a solo bike---until you encounter deep sand or mud. And in deep sand or mud even the widest knobby will be inadequate.
While the strength of a 26-inch wheel allows greater versatility, the trade- off is speed. I estimate the speed difference with otherwise-identical tires mounted on same-width 700c and 26-inch rims will be nearly 1 mile per hour. Thus, on a 26-inch wheel tandem you might expect to finish a century 15-20 minutes slower. If your ultimate goal is a first place finish on your club's annual century run; 700c is as necessary for you as a 26-inch wheel is necessary for a couple whose aspiration is to ride goat paths across the Alps.
Most couples find themselves between the above extremes. If your nirvana is neither a jeep trail nor a four-hour century, which size is best?
If you rarely ride with a group and your loaded tours are apt to include long stretches of graded dirt, 26-inch wheels are clearly the better choice.
Because most couples rarely ride their tandem on dirt, and often find themselves riding with other tandem couples or packs of younger riders on singles, 700c will rightly remain the more-popular choice among tandem enthusiasts.
Over two-thirds of tandeming couples who become disenchanted with their initial choice and "trade" from one wheel size to the other are switching from 26-inch to 700c.
Because there is no best answer, an intelligent goal is to own at least one tandem with each wheel size.
P.S. For each of the past five years, Jan and I have ridden "her" 26-inch tandem more often and "our" 700c tandem more miles.