My first question would be, are they both fixed? If not, getting him neutered, and her spayed will help tremendously.
Have there been any changes in the household recently? Even little changes can affect cats, they really like the same routine all of the time.
It is getting on to kitty season. It could be that you are being visited by strays and ferals, and they are marking the area around your home, causing this behavior in your two cats. You can go to a local beauty salon and ask them for a plastic baggie of human hair, then spread the hair around the yard, to keep the wild ones away OR you can start feeding the outside kitties, give them shelter, get them trapped, neutered and re-released before kitten season is in full swing.
Buy a couple of cat ramps and put them up on the wall, high enough for the dominant cat to sit there and feel secure.
Play classical music in the background softly, it really does help, and for some reason classical harp music works the best.
Invest in the new product by Feliway, the Comfort Zone room plug in.
Add Rescue Remedy to their water or food to help calm them, and if you have a certain scent you wear, spray your hands with perfume, shake them out till it dries, then rub each cat down with your hands to transfer the scent.