The answers here explain how to make a slider in the GUI which updates the figure whenever the slider position changes, instead of only when the user lets go of the slider.
You can link the slider value to an x offset so that one of the plots is shifted by that amount when the slider is moved. If you need to be able to grab the points in the figure window, try these answers.
Finally, if the only positions you want to allow are the actual plot and the shifted plot, why not include a switch or button instead of a slider? Those should update near-instantly and won't allow users to set anything in between the two possibilities you want.
