Need or want?
Hey if you 'feel' safer (mentally) with discs get 'em. If not, don't.
Over 220,000 miles on tandems so far. Tandemed at over 9,200 ft elevation. Have done long/steep/twisting descents off mountains. NEVER had an overheating problem. Much has to do in 'how' you brake.
Have used the old Mafac cantilevers ('70s and '80s), U-brakes, Self-energizing and V-brakes.
Have test ridden tandems with discs . . . they are fine. We are happy with Dura Ace front caliper and Tektro Mini-V rear brake on our current Zona tandem.
All brakes can have their own issues . .
Have ridden tandems with discs . . . and there are all sorts of brands/quality/ discs out there.
So please yourself and get what you feel comfortable with.
Just our input/experience.
Pedal on TWOgether!
Rudy and Kay/zonatandem