I agree that that particular species of pleco is not a good choice for combining with polypterus, but those marks were not made by a pleco. Those look to be physical injuries. (from digging, sparing with tankmates, etc.) But I'd say there's a fair chance that both polys have become infected with some sort of bacterial something. If they're still eating that's huge. That way you can treat their food with medication so it goes directly into their bodies. If it's not eating, you'll have to dose the whole water column and the chances of recovery are significantly reduced. I have treated several of my polys with sulfathiazole, but most general antibiotics should work.
And as always get the water quality to perfect, escpecially while these polys are sick. And I'm situations like this i prefer Epsom salt to rock salt. (1 tablespoon per 10gal)