 It seems to me that the dog is getting attention at that point, being that they all go outside anyway. The dog's getting attention at that point. That's what's causing the behavior issue. Easily resolved. I've had cats that would hate when the dog goes out. Dogs would hate when one of the dogs goes out and they have to stay in. All those things, just like kids, you just have to work it out and make sure that they don't think that one's getting more attention or different attention than the other.