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Chuck-
 I looked at my sextant after reading your message (can only take sights on 
Canadian Forest Fire Smoke tonight) and realize that although both the index 
and horizon mirrors do have a "tilt" adjustment, the two have their 
adjustment screws set at different points (i.e. one at 3 o'clock, the other 
near 8 o'clock) so that no matter how the two are adjusted, there is really 
no provision for making them completely co-planar in left-right alignment. (I 
*think* my geometry and physics are good enough to be sure of that.)
 That would mean that unless you can find a particular model of sextant where 
the tilt alignment screws are both at 3 or 9 o'clock, you would have to 
depend on the alignment of the frame as a whole--or use a split mirror, or 
place a vertical mark on your whole mirror, such that you are only working on 
one vertical line (i.e. where a split would be) rather than across the whole 
width of the mirror.
 I would think that a craftsman with very good hands could either adjust one 
mirror frame, or (safer) shim the mirror out as needed to try making it more 
fully parallel to the other mirror.

I wonder if this error is simply inherent to whole mirrors and ignored in 
them, or if any maker has taken pains to allow it to be corrected out?

   

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