For the longest time I practiced with these two mirrors on adjacent walls, but it wasn't until I actually hung up the mirror on the right (rendering it useless for anything but practicing), and turned my chair and stand at an angle so that I can see both images without the music stand getting in the way (why did it take me so long to figure this out?), I never reaped the huge benefits of being able to see what is happening on both sides of the instrument (in real time) while I am practicing.
One of the benefits of teaching is seeing the incredible difference it makes to the sound when something a student does looks right (a hand position, a bow hold, the place of the bow on the string, the direction and speed of the bow stroke). This configuration allows me to see the right side of my instrument when I am in upper positions (in the left mirror my hand is always in the way), see my vibrato, watch my shifts, notice when a lower string is being pulled rather than being cleanly stopped, see bow changes, and see the lapses from "ideal" that I miss every so often. Much more often, it seems, than I had imagined. It helps me to be a better teacher to myself.
Here is the operative geometry: both mirrors are a the height where I can see my whole playing mechanism while sitting down, which is what works for my particular corner because of the windows. The larger mirror is on the right, and it is 23" wide (minus the frame). The smaller mirror on the left has 19.5" of mirror (minus the frame). The distance from the corner of the room to the 2.5" frame of the right mirror is 28.5", and the distance from the corner to the 2" frame of the left mirror is 45.5 inches.
Here's a drawing of the missing violist (made by our son Ben during a concert I played in February of 1999). He was nine at the time. I was 39.