I still use two mirrors as I'm still too scared. I tried without it a couple of times and was pretty successful until I shaved over a bump on the back of my head. It hurt like hell and I had to shave around it for a long time
If I do shave at the sink, which is very rare, I use the one mirror and go by feel for the back.
Not hard to do once you get used to it.
I am a bath or shower guy, and go completely by touch and feel. Works great for me!
I use a mirror at a sink that has this cool trifold in it.
I can turn the two outer sides in a little, look in one to the other and see the back of my head perfectly! I suggest you try it some time! haha