Ugh. Hard one then. Parusing the internet it seems from here to be a UK and CA only product, and the company websites sells only complete kits. They do have contact info though so you may be able to arrange something.
Kind of surprising. Usually manufacturers will sell separate parts.
Probably the easiest thing to do is buy another identical kit as was mentioned by
@Toronnah . Not a perfect solution but at least you could have one charging while you vaped the other. The cable seems to be straight USB with no regulation, so it might be possible to take an old USB cable, cut the end off, identify and strip the power leads, and engineer something that functions. Lot of work though and it’s an iffy solution.