I'll give it a try. Already have. There is a tendency for the two images - the clock and the thumb to drift together after a moment or two. I can see that concentration can keep them apart. I wonder about this but I'm certainly willing to give it a try. My competition experience is in Bullseye shooting, both conventional pistol and international. I normally shoot with both eyes open.
Instructors and gun writers aside, there is a wealth of evidence from the "optical community" that supports the sub/unconscious shift from eye to eye though. I'll work on it.