I had the same problem. To do this, my wife held both dogs by their collars. First, I commanded "stay". Then, whenever I called one, my wife released him while firmly holding on to the other dog. We repeated these over two days, and finally it began to work. Now we can call one at a time, if we have them to stay first (otherwise, they both come!). It was frustratung in the beginning but with patience it really worked.