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Hi Logan
To make a perfectly flat (optically flat) surface, you need three blanks and they have to be regularly flipped top to bottom and swapped in and out of the pair.
I am understanding that you mean you are grinding the back of the mirror flat? Unless it is really uneven, it's not necessary to flatten the back.
However if you want to do it, then yes, do flip the 2 mirrors otherwise the top one will become concave. Flipping two mirrors, will make them roughly flat which is all you need.
Cheers
Joe
Last edited by OzEclipse; 28-04-2018 at 07:13 PM.
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