I used to aim with just one eye open. While I shoot for fun, it is nearly always in the back of my mind that I might someday need to use a gun for self-defense. So, some years ago I decided to train to shoot with both eyes open. It doesn't make aiming easier, it is to provide the full field of vision in case the attacker isn't alone and someone is coming at me from my left. I want to be able to see them coming. That said, I do sometimes revert to old habits and while I'm shooting, sometimes my left eye ends up closing. I find it has little to no effect on my aim (only my peripheral vision). Like JohnKSa said, even when you aim with both eyes open, you are still primarily relying on your dominant eye to do the actual aiming. Line the sights up with your dominant eye, have the other open in order to provide a full field of view.