A little context: the dogs have been together for 2 years; until 6 months ago, we had a 3rd dog, an ancient husky mix. When she got quite old and feeble, Vanya also started guarding his bed area and me against her, and I dealt with that purely by management, so my ancient Juneau would never have to worry about him. Before she got feeble, they played together politely for the most part, so this seemed age-related, because Juneau was always very deferential and careful around the young male Vanya.
Tiva, however, is another matter. She's old (14), but still full of energy and she has no intention of being pushed around by the young punk. The two dogs used to play for hours outside, until Tiva got cancer 18 months ago, and she stopped wanting to play with Vanya at all. In the past month, much to my surprise, she has started initiating outside play with him again (under my close supervision), and they get along well outside.
Inside, they'll never challenge each other for a spot once they're settled, and Vanya is perfectly polite to Tiva most of the time. But if Tiva gets off her spot on the couch and wanders over to the kitchen, or bumps into Vanya's bed, he'll snap at her. (She doesn't turn her head away, as Juneau used to do--being another pibble, even an ancient one, she would get into a full-fledged display. We avoid all this by trying to be super-vigilant).
I spend a lot of time now training both of them together, and they seem to love it, and they're perfectly polite side by side, waiting their turns for the treats. I also enlist my husband's help playing Look at That with Vanya: Frank brings Tiva closer to where Vanya and I are hanging out, and as Tiva gets closer, Vanya plays LAT and gets lots of cheese for remaining calm and relaxed as Tiva approaches. When Tiva goes to the water bowl, I play LAT with Vanya. When Tiva goes to the stairs, I play LAT with Vanya. When Tiva goes to the door--you get the idea.
My sense is that I just need to keep playing this LAT game in various contexts that Vanya might feel the need to resource guard, and also continue using baby-gate management techniques when we all want to relax.
(Over the 2 years, they've had a number of noisy displays that never involvedany teeth contact, until several months ago when a guest plunked a pile of chicken parts on the ground right in between the two dogs, and Vanya bit Tiva, resulting in a few stitches. That's when the resource guarding began, and that's also when all the baby gates and LAT began. Interestingly enough, a month after I started playing all the LAT games with Vanya, Tiva started playing with Vanya again--so my guess is that, even if LAT hasn't 'solved' the indoor guarding, it has relaxed them enough so that Tiva would choose to start playing with Vanya outside. If Vanya tries to initiate play outside and Tiva turns her head away, Vanya will politely trot off and play with something else instead)