That is correct.
What this does not factor in though is a function of the brain called "Summation" and you can read about that in the posts that Edz made on binoviewing.
To summarize his posts, while the single eye only gets the 50% of the brightness that the eye would get in monocular vision, when the brain gets the two signals, it adds them together in such a way that the brightness is perceived as brighter than each eye alone see.
This means that when binoviewing, a target will appear about as bright as it would be in a telescope of about 70% of the aperture used at the same exit pupil. (or something like this... You can get the exact details in the article on summation).
Also, the surface brightness of extended objects can be made brighter simply by using slightly longer focal length eyepieces.. In other words, rather than using a 10mm pair of eyepieces, which would give a dimmer view than a single 10mm eyepiece, one could use two 13mm eyepeices and get about the same brightness, though at a slightly reduced scale. Now most people report that things look bigger when using both eyes, so this reduced scale actually does not seem to be a problem in normal use.
Your math is correct though, but you are only seeing part of the equation. To see the rest of the equation, you may want to read Edz's post on binocular summation: https://www.cloudyni...nd-information/
Edited by Eddgie, 09 August 2017 - 09:36 AM.