I do this every time, and she still does it. I've been doing this about 20 times a day every day for about 2 months, and nothing has changed.
Well then something needs to change

It could help to have a trainer come over and watch what you're doing or even post a video for us to see. Sometimes a slight change in posture or your rate of reinforcement, or something else can make a huge difference.
As for boundary training, you would first have to teach it to just one dog when the other dog is outside or kenneled so they can't interfere. Work to the point where you can toss treats past their set line or even dump out their food dish and the dog still doesn't break. Only then would I try it while working with a second dog.
It sounds to me that your dogs might need more practice with impulse control in general before you add in the competition of another dog. Can you sit on the floor with a box of pizza and block the dogs from at least a 3ft radius away from you?
I better start working on a good down stay !
Would I also be correct in thinking that it would be good to teach them not to pay attention to the treats - I saw a video where they had a bowl of food and only treated the dog when it showed no interest. Would that help too ?
I think all dogs should learn that staring at the food doesn't get them it. The first game I teach is the "windmill" game where I hold a treat out to the side, dog stares at treat, I wait. Dog looks at me, I click and give them the food. I work to the point where I can move my hand around and they don't stare at it, and even hold the food dish out to the side and they don't stare at it. At that point when I work on training them with their dinner I actually put the food dish on the floor and train. At first I just reward eye contact with me, then being able to walk past the dish, then finally doing other behaviors, all the while I'm grabbing food out of the dish as the reward. I focus on guarding the dish by blocking the dog with my body rather then trying to physically grab the dog away.
I think if you can get to the point where you can train new tricks with the food dish sitting on the floor, it will be way easier to start training with another dog in the room. The 2nd dog will already have respect for the food dish to know they can't grab it, and will know that staring and mobbing your food hand never works.