Tri-view or bi-view mirrors?

I am doing a full bathroom remodel and the subject of medicine cabinets has come up. I am looking to make sure I have a proper viewing area for seeing the back of my own head really well, as I like to trim back there between haricuts. I used to have a tri-view medicine cabinet which worked well. However, for reasons that would bore you, putting in a tri-view medicine cabinet is difficult in the remodeled bathroom. What I am able to do is effectively have a bi- view mirror by using a single 24" wide medicine cabinet and putting a mirror on a nearby, perpendicular wall, so that when the medicine cabinet is open, it is effectively like a tri-view system except without the middle mirror.
My question is, how important is that middle mirror in being able to effectively see the backside of my hide. If I am looking into a mirror that is parallel with a mirror behind my head, will my head be blocking my viewline? I've seen many tri-view mirrors that I can try out but there's nothing in any local stores for me to try to test the bi-view design without actually buying two large mirrors and taking them home to test. Any help?
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