Marking Behavior With More Than 1 Dog
Your Pom that is marking is most likely doing so, because he or she feels the need to "claim" the house. Why? Because all dogs, of any dog breed, need to know the "order of the pack".
Long ago, dogs ran in packs and there was always a leader. Now, even though they are domesticated, dogs still need to know 'Who is in the pack?' and 'Who is the leader?' (also called the "Alpha dog"). If their human family member is the top leader, who is the leader among the dogs?
By feeding your dogs, teaching them commands, taking them for walks and all of the care that you give, you have already shown them that in the pack, it is you who is the top leader.
However, in a home with more than 1 animal, your dog that is marking, needs to know who is the leader in the sub-group of the animals.
Usually, dogs will try to figure this out among themselves. However, doing so can be very stressful for all dogs involved. This can lead to behavioral problems, such as this one.
You can help by establishing which dog is the Alpha Dog. It is usually the older dog. However, you can take notice when the dogs are playing. Is one of them more outgoing? Is one dog more "pushy" when it comes to choosing toys? Which dog runs to their food first? Noticing this, will help you know who is trying harder to be the Alpha Dog.
Once you know, you can then help both dogs. Remember that the dog that is not the Alpha Dog is just as important and loved as the other dog. Not being the Alpha Dog is not a negative thing. Both dogs will be less stressed and happy, knowing their place in the "pack".
When it is time to feed your dogs dinner, give the Alpha Dog their food first. When it is time to take the dogs outside for a walk, put the leash or harness on the Alpha Dog first. These small gestures help the dogs feel secure that you, the main leader, are showing them that you understand the "pack".
Once the dogs stop competing, they should stop marking.