I cannot shoot with both eyes open without a fiberoptic front sight that occludes vision of the bead to the non-shooting eye. Like the Uni-Dot.
To me that coach sounds like an ignorant jackass making blanket statements like that, because he obviously does not take things like the above into consideration. Find another coach who will HELP you and WORK with you, not try to make you fit to HIS preconceived biased opinion of how you should shoot.