Sadly, without a lot more information about which rim, where the split occurred in each instance, the team's weight, how much total weight is being carried on the bike and so on it is even hard to venture a guess.

Again, there are hundreds of thousands of Velocity rims out there that have been providing their owners with thousands of trouble-free service. We've used the 26" AeroHeat rim (same extrusion as Dyad) since '2000 on our full-suspension off-road tandems which, admittedly, don't use rim brakes, but that take a pounding on aggressive downhill runs over rock fields and the like. We've probably logged over 50k miles on Deep-V rims over the years and, again, no issues.

So, given that you and your friends are doing 'trans-type' tours the questions that need to be explored are germane to your specific applications: where are the rims splitting (e.g., radial splits along the brake track or down the center due to spoke pull-through, etc), how much weight is being loaded on the bike (total: front & rear distribution), which rims are failing (front, rear or both), and does it occur when riding on relatively smooth roads, or after riding on unimproved, rutted roads (is it fatigue related or caused by blunt force). If spokes are pulling through, who is building the wheels and how much spoke tension are they putting into the network? Again, there are a lot of variables to consider even in your specific trekking / trans touring examples.

Finally, what else are you hearing in terms of feedback as I'm sure you've posted out this same question to various other forums, tandem related and otherwise?