Have been riding 'in tandem' since 1975 and pedaled over 250,000 (quarter million) miles as a duo.
Have used all sorts of brakes; cantilevers, center pulls. side pulls, rear U-break and even drum and disc brakes (front and rear).
All have their advantages, all let you stop . . . . some better than others.
Our personal preference for the past 40,000+ miles is Dura Ace sidepull front and and mini V-brake on the rear.
We are a rather light duo (sub 250 lbs total).
Have descended Kitt Peak in AZ (a 6%+ grade) and 11 miles long with lots of switchbacks in the late 1970s with Mafac cantis front and rear and NO 3rd brake. Worked great!
Did have to make one stop as pilot's fingers were cramping up from continuous front/rear braking lever use. Made a 5 minute stop and felt both rims. They were NOT hot, just warm. Continued our descent safely.
Have descended more steep/long hills (Example: Yarnell hill between Prescott and Congress AZ and topped off at 50 mph near the end) and in many other states, using many other rim brakes.
Test rode disc brakes on a custom tandem when discs first appeared and felt that they were overkill.
Stories of tires blown off overheated rims and plastic disc parts melting are legion.
Have also used the old drum brake setup and felt that for us, it was not necessary.
Just our experience and input. Your needs/wants may very from ours. Do what is good and comfortable for your team.
Pedal on TWOgether!
Rudy and Kay/zonatandem