2-month-old guinea pig screams sometimes

The owner bought a pair of 2-month-old guinea pigs and housed them together. One was screaming now and then but both were eating.

"Do you have Revolution? Are they having skin mites?" the mum asked. I put the guinea pig onto a piece of white paper and rustled the body hair. No hair dropped off and 2 or 3 black specks. The black specks were not mites as seen under the microscope.

"No mites in the body," I put a thin cotton bud into Alice's ears. "Black dirty wax was seen and the guinea pig squealed."  When I did this on the other pig, there was no pain response. Both were bought from different sellers.  

"Do they use their hind legs to scratch the sides of the body?" I asked.
"Yes, Yes," mum said.
"This is due to ear pain or infections," I explained that cats and dogs with ear infections do it consistently.

I advised ear cleaning 3X/week with the ear ointment containing insectide  and separation of the two for one month with a grated barrier such that they can smell and see each other, to prevent bullying.

Overall the guinea pigs look thin and so should be alone to put on weight