Have broken maybe 4 timing chains in our 225,000+ miles of pedaling TWOgether.
Majority of the time stoker 'powered up' and got us home; another time it happened on a 5-day tour and chain broke in motel parking lot and had spare links/tool in our luggage..
However on my single (9-speed) utilized a chaintool and broke about 4 chains in a matter of months.
Never used to happen with 5 - 6- or 7 speed chains.
Get around 3 times the wear out of a crossover than drivechain on the tandem; but then we' are quite picky about keeping chains clean/waxed.
Pedal on!
Rudy and Kay/zonatandem