kevin wrote:
One trick from woodworking (or somewhere... I can't recall where it is
that I used this) is to grind the two pieces against each other. No
need for a reference "straight" piece -- the two will self-correct for
any non-straightness.

But it sounds like that would be awfully hard to do with glass by hand.

No guarantee they'll be straight, that way. They can have an equal
curve, and still fit each other.
Afair, the early toolmakers used to lap *three* straight-edges against
each other (in all possible pairings). That way, they have to come out flat.