A stacked focus acts as both kinds of focus. In the example you gave, of a Force 4 Power/Force 4 Summoning stacked focus, the Summoning side would be redundant. Really, if one side of a focus is a Power focus, the only things you'd be able to use on the other side would be things like a Weapon focus, Centering focus, and the like. In other words, the ones that apply to all those tests that don't involve the Magic attribute.