I just remembered one thing that helped me with the grand-dogs is walking; it really seems to help. I know Cesar recommends it, and I guess it helps the dogs feel like they are part of something bigger than themselves. I'd use the word "pack", but people attack when I use it!
Davinci may be a little young yet, but I'd definitely try walking them together. It also wears them out, and they say a tired dog is a happy dog.