To answer your question, I'd find a craigslist ad for a set of Tandem wheels built on a 48 hole hub, and have them gone over at the local bike shop (retensioned/straightened), or buy a 40 hole hub (Raleigh rear wheels once used 40 hole hubs on the rear) and build it with a new (alloy) rim, from Peter White. If you break spokes they will guarantee the wheel for you, if you haven't misused it. Make sure these are the widest/wheels and tires that will fit your frame and/or fork, and mount a good quality fat tire on them. The fat tire absorbs most bumps, which cause much cycling of the load on your spokes. Not to mention if your tires are low volume, be sure that you are not exceeding the load rating of the tires. Sure, heavier gage spokes are good, but more of them are even better, and a great tire is best. Oh, and go to a smaller rim if spokes are breaking regularly. Going down a size makes the same number of spokes stronger, 10% is the usual guesstimate. (700/29" to 26", 26 -24, 24 -20, 20 -16).