When shooting my red dot equipped AR's under about 50 yds I naturally shoot with both eyes open without difficulty. Never tried it any further.
My old man has a vintage Single Point sight on an M1 carbine that the objective end is a glass bubble with a angled red plastic piece inside. It lets a little light in to form an red "dot" but you can't actually see through it. Works fine with both eyes open as long as you don't wait to long to fire or the dot will start moving
Review of Single Point with pics
http://shootery.blogspot.com/2010/07/moldy-oldie-review-singlepoint-sight.html?m=1 
