Tandem braking is a different beast to half bikes, where weight transfer means that most of the braking is done by the front wheel as the back becomes unloaded and thus locks up much more easily. With the long wheelbase and rearward weight distribution the effect is much smaller on a tandem - it's impossible to do an endo, you'd lock the front wheel first. So the brake bias is much more towards 50:50, and with disks you just want the biggest you can get as physics dictates that you'll be going 50% faster with twice the weight to stop.
Not that we were doing much braking. Big roads with good sightlines and good surfaces meant pretty much all the descents could be taken flat out and 50mph+ was the order of the day. Made up for the long upwards slog.