I did find this website and he has an interesting way of making two mirrors that will end up being the same.

http://lerch.no-ip.com/atm/

"Since I’m building a Binocular telescope I need two mirrors. To make it more fun each mirror must be exactly the same! To do this I ground both mirrors on the same tool, which worked amazingly well! To implement this I would put down some grit on the tool, Put mirror A on top, do 2 laps around the barrel, with 20 cordial strokes per position, and 8 positions around the barrel to make 1 lap. After completing 2 laps I would set aside mirror A, add fresh grit to the tool, put mirror B on top and repeat."

Was it luck or will that actually work?