Is Katie fixed? or Elvis? Katie is simply marking. This is how female dogs communicate to male dogs. They "pee" so the male dog will pick up their scent. The male dog can tell if the female dog is in heat. This may still occur if she is fixed because it is normal dog behavior. It is kind of like them smelling each other behinds.
This behavior should end as they get used to each other. If it is bothersome them think about getting some "Hot pants". They are for female dogs in heat and work like pads. You can find them at the pet store. Good luck! and get a steam cleaner!!!