The single biggest point of failure for us on our mtb tandems (and we don't generally ride loaded) is the rear hub. Get a good one upfront. It will be the best $$ spent. Chris King, DT Swiss, Hadley or Onyx are noted to be some of the most durable for tandem use. We only use Chris King rear hubs on the tandem. Pushing your bike b/c you hub spins freely in both directions is not fun.

We have destroyed a few rear hubs, broken a few spokes (but not full wheel failure). So carrying a few extra spokes in the correct length and knowing how to fix/true a wheel would be good. Also, chains can break, so bring extra links and know how to install. Doing a ride that long, you will likely have to install a new chain during the ride (due to wear) and likely tires too, so plan a stop mid-way (Northern Colorado or Wyoming) at a shop or ship a box somewhere.

I can't speak to the Double Edge Dawes bike or its durability.

Again best of luck and we want to hear about your adventures!