It depends on your team weight, and your riding style. The problem with braking on a tandem on long descents is not stopping power, but, rims over heating.

A lot of the descents in Colorado while long, and reasonably steep, are not terribly twisty. So if you're comfortable with letting the bike run between the corners, and only braking for the turns, you may well not need a drag brake.

If you want to descend more slowly, and have the breaks on pretty constantly to do so, you would benefit from a drum brake.

Personally, I would go ride it without the drum brake and see how it goes. The worst that's going to happen is that you'll need to stop and let your rims cool. And you'll know whether you want the drum brake.