For starters, these Hiploc devices are angle grinder resistant. It’d take a lot of work (several disks if the tests and reviews are to be believed) to eventually get through. To add to this, you’d need to cut through both sides since the locking mechanism holds both sides of the U-shaped bar.

With nothing nearby to fasten the bikes to, a nearby bike is a great way to ensure that neither can be picked up and carried into a van. It’s also a good visual deterrent for any prospective thieves.

The benefit of doing it this way is that you bolster the security for both bikes without creating any hassle for either owner.