When you leave your tandem locked anywhere, remove the front wheel and take it with you, whether in a store, restaurant or even your apartment.
A little less easy for thief to wheel a tandem around on just it's back wheel (yes, have done it getting a tandem inside a small elevator).
They can still steal it but make the task a bit more difficult as they cannot just ride away on it, but can still toss it in a van/truck.
Bringing tandem inside and up a 3rd story is a bit of a challenge and yes, have done that a few times; ask apartment manager it there is a storage place in the building, like maybe where they keep stuff, and offer to pay a small fee for it's use.
We are fortunate to have a garage to park car/bikes in. We do use an extra long cable with small titanium combo lock that we utilize while we are out-and-about; we thread the cable through both wheels and usually a stationary object; however that only discourages joy riders and a non-profession bike thief.
Any lock/cable can be defeated by a pro.
Pedal on!
Rudy and Kay/zonatandem