Our 4 1/2 month old german short haired pointer/terrier mix Elmo, at 4 months, was basically housebroken; then another dog came into the house (we are dog watching for the next few weeks) and suddenly Elmo is peeing everywhere, in very deliberate spots, like on the couch when we are sitting with him, or right between us and the other dog. Definite issues of territoriality and ownership going on, but not sure what to do about it? Any advice?

You may want to try correcting this behavior with vinegar and water. Every time he pees, squirt him in the face with a stern no.. Then escort the dog outside and have him go potty out there... praise him for going outside.

It might be a case of giving Elmo a little more attention.
Make sure you have some good one on one time with him so he doesn't think that the 'new dog' is taking over.

When you see him urinate in the house give him a very firm "no" or "bad dog" and take him outside.
The squirty bottle is a good idea, I would do away with the vinegar though, just in case it gets in his eyes.
Any spots where either dog has urinated should be cleaned up with an enzyme cleaner. Enzyme cleaners completely break down the urine and completely removes the smell.

You need to make sure your other dog does not feel abandoned.
Try things like.. always feed your older dog first. Greet her first, etc etc so she doesn't feel like she's coming in second even though she's been around longer.

Follow the others advice. Alternately you can bring a first-morning urine sample into your vet to make sure it's not a UTI with just bad timing, but it sounds more behavioural to me.