How To Make One Usable Bike From Two Perfectly Good Ones…
Pick a high crime area, and use a single mini U-lock to tether two bikes, while you head off for dinner.
(via Cyclelicious )
When you return, the chances are that the front wheels of both bikes will be missing, though with any luck you’ll still have the rear ones. So if you can somehow manage to get one of these into the forks of the other bike, at least one of you will be able to ride home.
As Richard Masoner points out, you’d do better to find a thinner pole, and lock to the rear wheels inside the rear triangle - effectively, a double Sheldon lock . The rear wheels would definitely both be there when you got back, though the front ones may well be available only on EBay / Craigs List / etc.
Or perhaps we’re only addressing the symptom here - the fact that they only have one lock between them.
So my top tip is to always take your lock with you; lock up using Sheldon’s method, and if your front wheel has a quick release skewer, take this off to secure it with the lock or take it with you. If your front wheel doesn’t have a QR skewer, you’re going to need a second lock to secure it!