As a dual and triple boxer, I often find I do click the mouse on the slave screens to ensure the right cast at the right moment, as having cast-sequences with blank entries can be stuffed up if you are slightly out of range, or a slave is facing the wrong way, etc. I use single PC with maximiser on spanning monitors, which means I have to click once to enable the window and then a second time on the spell that I am after, but you get used to the double click and everything works quite well with practice.