He is marking HIS territory because you brought the two pups into HIS house. Some dogs are very much into marking their territory and telling other dogs it is theirs. I have a female that if I don't watch her like a hawk will mark in the house. It doesn't have anything to do with being house broken, peeing and marking are two different things. The only thing you can do is be very viglent and when you see him going to mark, reprimand him.
My dog did this too when my first foster came, she was quite dominant and a wannabe alpha. With my subsequent fosters he has not done it, but they were more submissive. He is not very dominant, and also I think he is getting used to the new furry friends that visit and he is feeling more secure (perhaps?).
Just keeping a close watch on him and their interaction helped me to prevent marking, also I think I had to ensure that I was paying him a little extra attention every so often to make up for having to tether the fosters to me to housebreak them. Without that he was perhaps feeling ousted?