That functionality doesn't exist, I'm curious though. What would be the point? Why not just use all in one? You would still need to click a Camera Icon or hit Enter or some keyboard shortcut before saving the capture window.
One thing that might help is that when you click and drag to select the area, if you are still holding your mouse button down and you want to abandon the selection thus far and re-select, you may press the Esc key to start over.