Heroes/henchies and targetting.

Recently someone told me that it's possible to specify a target for a single hero or two while my character attacks another. They said that there is a targetting reticle that can be used to do this. I cannot find this and would love to know exactly how this is done? Can anyone explain this to me precisely? The only thing I can do so far is to have all the heroes/henchies hit one target while I alone attack a different one.

for heroes, if you bring up the skill panel for the hero in question (the number beside their name), you'll see a button that looks like a circle with a cross through it, clicking that will lock them onto whatever your current target is.