Locks/chains only keep honest people from stealing or going for a joyride on your tandem.
Having said that, we carry a cheapy and short plastic coated cable lock that uses a coded credit card to unpop the lock. Bought it years ago when one of the catalogs was having a clearance sale.
Lock the bike to something (post, bikerack, fence) . . . and loop cable through back wheel/frame. This is only a deterrent and not a guarantee that someone will not or can not steal it. Most locks/cables can be cut/busted; pros use cable cutters; some use freon to chill the lock and shatter it. If they want it bad enough and are good enough, consider it GONE!
OK, now you say what about my front wheel? If you feel so inclined use a separate cable/lock for that wheel/frame lock up. That way prospective thief has to bust 2 locks/cables to steal the bike. No need to get a super long cable with one lock; 2 locks and 2 shorter cables are more of a deterrent.
Given opportunity and time and tools, any lock/cable/chain can eventually be busted.
Homeowners insurance will sometimes cover the cost of theft if bike was locked properly.
Now lets go out and ride . . .
Pedal on TWOgether!
Rudy and Kay/zonatandem