Scratching Itching, dry skin (i think)

Hey all, recently one of my guinea pigs has started to itch a lot, i thought it was because of the hay i was giving then and the fact that it was a little more course then the stuff i had been giving them, so i figured that is the reason he was becoming itchy. The second guinea pig does not itch as much as the first. I ended up giving both of them baths and found that the first one had a little scab on his butt in one of the places that he itches, and that there were some areas where his skin was flaked and dried out.

I cleaned their cage before their baths and didnt put anymore of the course hay in there (which they werent happy about) but from what i can tell he is still itching. I was thinking it might have something to do with the heat and that i should get a humidifier for the room.

If it was mites, wouldnt both of them have it as opposed to just 1?