I have a regular bike security cable, pretty heavy duty. It’s the kind with loops built into each end. I wrap it through both bike frames, both wheels, and then down to the receiver hitch, where I lock it with a big padlock.
I have no illusions that this will stop a determined thug who comes with tools. But it will foil the casual thief just looking for a target of opportunity. It’s all about not leaving your bikes as low hanging fruit.