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?