I think the brain training required is very similar but probably not quite as demanding as the Apache 'bag' training and I know that adapting to it is a very individual issue. I will check the actual eye relief on our NVG when I get back to work.
It is not unlike using a camcorder with both eyes open but one looking down the viewfinder - I had to use this technique whilst skiing and videoing my kids so that I didn't crash or fall over!
It's like so many things in the aviation world, it can be done but to do it safely requires training and practise.
It might be that I favour larger eye-relief because my eyes are getting older and I'm more used to dealing with a smaller retinal image as I have to hold books further away to focus!!