I am hesitant to claim to be qualified to answer the question under any circumstances, but for the benefit of others who may be, I want to qualify a few issues:
1. Where do you ride in Colorado?
2. What is the total weight of bike and team together?
Without this information it is probably impossible to assess the degree to which a hub brake would be preferable to, or a good adjunct to, a rim brake. We want to know if you do steep descents with switchbacks.
There is a separate debate with regard to whether or not the Avid can qualify as a functional drag brake, owing to pad size and disk size. I'm not going to touch that one either.....
The double-pivot caliper brakes do not necessarily accept all tire sizes that are likely to be used on tandems. Hammering a (nominal) 32mm tire through the pads on Ultegra sidepulls is just about the upper limit.
The function of a "drag" hub brake is to convert the potential energy of height into heat without running it through the rim, where it might cause various types of tube and tire failures. One might suppose that if your rims are not getting noticeably hot (and of course you do check?), you don't need a hub brake. Until tomorrow, when you try a new route.
My notion of responsible captaining verges on paranoia. My stokers like things that work ALL the time. Tires that never go flat (so I use kevlar-belted tires), chains that never drop when going for the triple (have N'Gear Jumpstop, or whatever you call that thingie that Peter White sells....), spokes that never break (use Aerospokes, so no wire spokes), wheels that never go out of true, captains that never screw up, etc., etc. I do my best to make decisions about componentry as though I were them with my body of knowledge. In country where I needed a drag brake, I know my choice would be equipment thought to be reliable with a long history behind it. Certainly a modern large disk is tempting in comparison with the Arai drum brake.