In all honesty, this is one of those questions/problems that no one will be able to help you with until they at least see some photos.
If you don't have a digital camera (no cell phone pics please - they are never clear enough).... either borrow one from a friend or purchase one of those disposable digital cameras.
If you are that worried about them it is always best to make an appointment with a herp vet. Be sure to ask the receptionist if the vet has experience with pythons and be VERY picky about the vet you go to. Don't just make an appointment with the first one that will see your animals.
However, since you just recently noticed this on not one but two of your animals I have to say that it most likely is not a kink (especially if it "comes and goes"
. Since they are still moving around and (I assume) the area is not tender to the touch or swollen I also doubt they have broken their backs (ribs maybe?).
Without pictures it's impossible to give you an accurate appraisal about what may (or may not) be going on. Right now the only two things that come to mind are either broken ribs (they can pop in and out and will allow your snake to maintain a certain level of mobility... however this is a very serious problem and should be dealt with by a vet a.s.a.p.) If they broke their ribs it could have resulted in a number of ways - someone dropped them or perhaps they were pinched somehow in their tubs/cages as a result of someone opening/closing the lid or tub or from a failed escape attempt.
The other option... your snakes are underfed or going into shed and what you are seeing is simply skin folds. Of course if they are overfed they will be pudgy and have folds also...
no pics = only guesses Sorry!