When I'm working with my crew, I put the others on downstays or in "places" (couch, chair, blanket..etc)while I'm working with an individual dog. Repetion works wonders.
If they get up or out of their "place" I just say "AHT!" and put them back. If that was too difficult for the other dog, then I'd leash them to their spot. Try to work so that its easiest for the dog to succeed than to fail. If something is too hard, take a step backwards and end on something the dog CAN do.
I always try and keep everyone within sight of the dog I'm working with, if I crated or locked them away they would get extremely upset. So I figured I'd kill 2 birds with one stone.
It helps doing it this way then they all learn to listen to you while they're together instead of paying attention to each other(a plus when having a 6-pack lol) Now I can play fetch and have them all stay while 1 fetches.. or 2 or 3... But it takes baby steps to get there.
An idea to help Stella to calm down would be to have a special bone, hoofie, chew or kong that she only gets when he's going out the door(it has to be extra amazing in her eyes to). Then when he comes back in, pick it up and put it away. If that doesn't work try a sound to break her attention and then when she looks to you, find a toy or something. The trick to training dogs isn't to get them to NOT do things, its to teach them what you want them to do instead.
The most important phrase I ever learned in dog training is my personal doggie motto.
"Interupt, redirect, & praise"
By doing that you can untrain just about any bad behavior.