Excellent advice
Thanks, all, this is very useful information. What we've learned is that:
A) No one is making drum brakes any more, and
B) disc brakes are not meant to be used as drag brakes, meaning that
C) drag brakes are not considered essential safety gear on a tandem. I had assumed it was a necessary piece of equipment.
With all this good information in mind, my wife and I will start conservatively and learn how to manage descents with the brakes already on the bike. If we just can't get to a point where we feel comfortable that we can control our downhill speed, we'll invest in a drum.