You can train yourself. When I first started it was hard to do but after a surprisingly(sp?) short time I found that I could do it, shoot better, and I still had my "side vision" with both eyes (I can't spell the per**** word). My aim as well as my shot placement vastly improved.
Yet, I know that some people can't do it (my best friend comes to mind). He did try and he stayed with it for a time but he just couldn't do it.