I shoot everything including practical pistol & rifle with both eyes open. I only close one eye when shooting weak-sided with a rifle, or sometimes when using a high powered rifle scope.
Which eye is used for the primary focus can be changed by your brain -- by thinking about it. Assume a shooting stance with whatever gun you're trying to learn the technique on (rifle, handgun), and then close the off eye. Notice what you're focussed on. Now open the off eye and retain the same focus. Eventually, you'll be able to selectively focus on the "correct" of the two images. For handguns, you should have the front sight in focus, unless you're doing something advanced for IPSC.