These two were set up - told the stay for a photo. The discomfort you see, is not due to not knowing each other but being set up too close to each other and told to stay with a camera pointed at them and more than one onlooker.
These two are the same age - one day apart and knew each other well. They went to puppy class together and the Toller came to the Parsons house for grooming.
Muzzle tight, cheeks tight, furrowed brows, ears back. Body is leaning away from the terrier. It is somewhat hard to notice this, until you look at the left leg. It is rolled slightly to the right and the elbow is pointed toward the terrier. The right arm is stretched out further and his whole body indicates he is ready to move away from the other dog, given the choice. He is listening to the stay cue, but not relaxed about it.
TIGHT = seeing visible lines around a specific area. The area may also look larger or more prominent (as in clenching jaw).
FURROWED BROW = lines between eyebrows, and eyebrows more prominent or raised.
She is very obviously leaning away from the toller. Her left leg is pointing to the left, away from the toller and her right paw is just slightly, partially raised - a clear indication that she is ready to move to the left away from the toller at any moment.
You can see her leg and shoulder muscles on the right quite clearly, which means they are tense and ready to move.
Her face is a little harder to read because of the longer hair, but you can see the same line under her eye as the toller, which means tension in her ceeks. Her lips are tight together and her ears are up and slightly to the side. If she were relaxed the fold of her ears would be more forward and closer to her skull. You cannot see her furrowed brows, but you can see the hair between her eyebrows is sticking up, which indicates a furrowed brow. She is also squinting her eyes, which is evident in her right eye. This is a calming signal, likely toward the person with the camera.