opthomologist (sp?) told me if one eye is shuit, it tells the brain it is dark and the other eye tells the brain that it is daylight creating a mild confusion. the pupil in the shut eye will dilate and the pupil in the open eye will dilate some in sympathy with the closed one, allowing too much light to enter the open one.
does that make sense the way i told it. anyway, i have shot with both eyes since i got my first BB gun in 1936. i have tried squinting the off eye, but as several have said, it is tiring. concentration is the key however you do it. good sight alignment and a good letoff makes for center hits. and, when you get a good sight picture, dont stand there admiring it, it will degrade, so press the trigger. this is advice on bullseye shooting, but i have dunked many little pesky critters over the years using the same system. i did switch from a six o'clock hold to a center hold after retiring to better hit varmints. i now wish i had used center hold all the way. all this excepting what the eyeball doctor said is just my opinions and observations.