I'm right handed, right eye dominant and have 20/20 vision. Everyone's eyes and brain work a little differently so YMMV, but here are some thoughts and suggestions.
In my experience, there are two main factors determining what you see with the ACOG - focus and illumination. When your brain is getting two completely different images from your eyes, it will want to default to your dominant eye, or the eye that is in focus, or the eye with the brighter image.
Typically that means if you are standing out in the open, looking through the scope with both eyes open at something you'll get a nice crisp image through the scope, with your peripheral vision from both eyes around that. I'll call this the magnified view.
If what you are looking at with the scope is very dark relative to your immediate surroundings, or blurry/out of focus (which can happen if you are moving or looking at something REALLY close), or you put a cover on the front of the scope, you will get the non-magnified view out of the non-scope eye with only the glowing reticle (assuming this is an illuminated ACOG) superimposed on your view. I'll call this the BAC view.
Between the two views you essentially have both a 4x scope and a 1x red dot. The challenge to be able to get the view you want, when you want it.
The magnified view is easy - look at the target with both eyes open through the scope, and if for some reason you aren't seeing the magnified view, just squint or close your off eye.
The BAC view can be more challenging. Looking through the scope with both eyes open, try to force your brain to focus on the non-magnified image. In some cases this will be very easy (depending on lighting and focus as mentioned above), but sometimes it will be very hard to force your brain to switch to the BAC view. Alternatively, you can shift your head slightly up or to the side (we're talking a few mm's here) - your view through the scope will start to blur and black out but you'll still retain the reticle and your brain will switch over to the BAC view. Once you practice this a bit you can easily and instantly switch between the two views on demand.
