When you use your aimpoint, you do not need to line it up with anything. Once it's zeroed, bring the dot to the target and fire. That's it. You don't need your aimpoint reticule to be anywhere near your irons.
Co-witnessing is just a convenient way of zeroing your aimpoint, which can be used independently. You don't need anything on there but the aimpoint, but back up irons sure are nice for when the battery dies mid fight.
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