These mites had to come from somewhere. I'm pretty sure these are species specific, so they had to come in from another snake. If you've had both of your snakes for some time, with no cross contamination from another snake, I don't see how they can be mites. Did you get a new wooden hide lately? I'm thinking maybe they were brought in with a piece of wood. If that's the case they aren't mites that will attach themselves to your snakes.

Good Luck!
Jim Smith