My computer is task impaired, so anything not immediately visual on kingsnake is not somthing I can check out.
Have you noticed any combative behavior between the two geckos? Both likely reasons to have swelling along the spine are related to trauma. One, is a broken back, which, in reptiles doesn't necessarily relate to paralysis. The other is an abscess, which is most often the result of a bite wound.
Spondylosis is often diet related and from what I can tell, your gecko's are eating properly. For this though, I am not a gecko expert.
For whatever is causing the problem, a veterinarian can help you out with a hands-on exam, and then X-rays. Blood work may compliment the diagnostics if the X-rays still leave differentials in question.
If your geckos are currently bright, alert, and eating/defecating well, than there is no rush to take them to the vet. I still recommend it before it becomes an urgent matter. If your geckos are acting ill, than a vet visit is immediately imperative.