"Consider this. I want to make a biplane from two identical wings of chord 4 inches. (draw it out). I place them one above the other, with the leading edges of both on the datum mark, and the AC of the combination is 1” aft of datum. By either method.

I now decide to mount the lower wing staggered aft one chord length, 4”. The mean chord is half way between the upper and lower wings. Its quarter chord point is 3” aft of datum.
By the method you advocate you will draw a chord 8” long and place the AC at 2” aft of the datum. That’s an inch too far back. That’s an error of 25% of the wing’s chord, quite a lot."
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You mean like this?