The Permalyn Sealer does penetrate the wood so it's not just sitting on top. Sanding back down to bare wood still leave (some of) it down in the wood, that's the idea of the stuff.
But the wood can be re-stained with the Laurel Mtn stains right over the sealer again. They just won't penetrate as much, or as quickly depending on how much sealer the wood soaked up, grain density, ect..
Other brands of solvent based stains will usually work over the sealer too..
Best to try on a scrap piece first sometimes to make sure it'll dry completely afterwards though.
I've had some uncompatable combinations (different brands) that just never dried right and had to take the surface down again.
No problem w/ using Permalyn/Laurel Mt over each other though.