Like Ridge Runner described, "Co-witnessing" is what you should be teaching them. The human brain is an amazing image processing computer - it's processing the proper overlap for two images at all times when your eyes are open.

So even if you're looking through a scope, your brain will try to overlap the images. If you're focused on the target with your left eye, and the right eye is in the scope, if you bring the crosshairs in your right eye to point at the target image in your LEFT eye, you'll actually be moving the crosshairs onto the target through the scope also. Strange how it works out that way, but it DOES work. It's called "co-witnessing".