Practice, practice, practice. Some people can do it, some never can.
I am right-handed, but left eye dominant. It took me quite a while to train myself to shoot with both eyes open, but I did it. I still have to use my left eye as the dominant one, though. Every once in a while though, my eyes/ brain get "confused".....and I can't seem to use both eyes for a few seconds....but this passes quickly.