I have tried numerous things to eliminate this problem. We did "Crate Games" (Susan Garrett DVD). I used to stuff Kongs and provide chewies... she just ignores them. I try to always train/exercise her first. Have her in view. Out of view. Closer to the action. It does not matter. If it is not her turn, you are going to hear about it.
I find the behavior stressful at trials because I cannot alter the running order. If Riley runs first, I might come back to Seven having a fit and then it is her turn. The last thing I want to do is to reinforce it by returning and getting her out. In the past, I have actually separated them completely so Seven is not triggered by the sight of me getting Riley out. But then I feel bad because I cannot sit by them both when I have down time.
So I came up with this solution and it has worked well at two trials. I cover the top and two sides of Seven's crate so she cannot see Riley. Seven has a view of a door that leads to the outside and away from agility. So she is fine with seeing me come and go through this door because she does not think I am doing agility. When it is Seven's turn to run, I come through the door she is watching and I take her in the opposite direction to agility. When Seven is returned to her crate, I exit the door she can see, even if I just run around to another she cannot see.
When it is Riley's turn, I come from the direction that Seven cannot see. Riley's things are always ready to go and I quietly grab her and go. Seven probably hears the movement, but is not triggered because she does not get a visual. After Riley's run, I quietly return her to her crate (out of Seven's view). If it is time to get Seven, I will exit through one of the out of sight doors and come through the door Seven is watching. Make sense?
It is labor intensive, but it has worked really well and best of all, I can sit with both hounds when I have down time.