We carry a short steel cable with tiny titanium combo lock. We lock up our tandem to something whenever we stop to shop or eat.
Had a smart***** bike club member tell us: 'I can still your tandem anytime I want to.' Told him: 'Do it now, you've got 3 minutes.' His reply: "All I need is cable cutters. . . ". Our reply: 'You've got 3 minutes". His: 'My cable cutters are at home." Nuf said!
While our lock/cable will not stop a pro, it will deter a snatch and grab opportunist.
Yes, have shared our lock with fellow riders if they ask.
Pedal on TWOgether!
Rudy and Kay/zonatandem