Is there a keyboard shortcut to select a window when multiple windows are open?

I frequently work with two or more windows open (Window -> Arrange -> Tile Vertically).  For example, when I'm soft proofing, the original image is in the left window and the proof copy is in the right window.   I frequently toggle back and forth between the two windows, e.g., to use the eyedropper tool.  In order to toggle, I must click a tab to make the window active.  It would be ideal if the active window automatically followed the tool.  But if that is not possible, a keyboard shortcut allowing me to select the active window with a keystroke would work almost as well.   Am I overlooking a feature that already exists?