Thanks so much for the lovely feedback.
Your best bet is to contact the guys at Toys4naughtyboys as they sell spares, although I just checked and the lock isn’t specifically listed, but email them, they’re super helpful, and I’m sure they can help.
Of course the other way to do it is buy another barrel lock cage from them or anywhere and the whole thing slides out and you can use it between the two cages. Unless you’re sharing with friends you’re never going to have to lock two at the same time, right?