Does smart casting prevent "Magic Box" casting? I other words, if I select two high templars and I cast psi storm on a map location instead of a unit, do both templars use their storm on the different spots in relation to their positions or does only one templar cast their storm?

Only one High Templar will cast at that location when you have a group of High Templars selected and cast Psi Storm. You'd have to give a second order to have another storm go off or chain the storms by using shift and then clicking the different locations you want the Psi Storms to hit.