In 225,000+ miles of tandeming since the mid 1970s, we have never (so far!) broken a drive chain in any of our tandems.
Did break about 4 or 5 cross over chains; stoker had to pedal us home while pilot steered.
Have used all sort of chains (except Campi or Wippers). Currently SRAM PC991 for drive chain and KMCs for crossover, all of them 9 speeds.
Agree, the narrower the chain the more likelyhood of failure.
So be prepared all you 10 and 11 speed afficionados!
Pedal on TWOgether!
Rudy and Kay/zonatandem