canopy2k,
Hi, I looked into this when researching for my book "Basic Aeronautics for Modellers". I did some working out and found some stuff on the web to back it up. That website has disappeared now (anyone know if 'The designers & builders online Workshop' has relocated somewhere?).
I don't know how to get my diagrams on the Forum but does this text help?

Biplanes
Use the following procedure to find the mean chord of a biplane, or a sesquiplane with unequal wings. For example the Fokker DVII has two unequal wings with normal (positive) stagger, i.e. the top wing is ahead of the bottom wing. I worked out that the top wing supplied 61% of the total wing area, and the bottom wing obviously 39%.
Referring now to Figure 22.7, I joined the mean chord lines of the top and bottom wings. I then divided the gap between the wings in the ratio of the wing areas, 39:61. The mean chord is 39% of the gap from the upper wing (the bigger one). (Some authors bias the mean chord towards the top wing but I have kept it simpler and made generous allowances elsewhere.)

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The CG formulae in Chapter 8 do not apply to biplanes because there is nearly twice as much downwash as the formulae allow for, and twice as much interference to the airflow, i.e. loss of airspeed over the tail. However, I have found that if you reduce both the Tail Volume and the wing Aspect Ratio by dividing them by the number of wings, then putting these reduced numbers into the original formula gives a sensible CG.
In the case of the Fokker, I called the top wing 1 and the lower wing is 0.64 of it, so the number of wings is 1.64. That gave a reduced AR of 3.3 and a reduced V-bar of 0.24 which in my usual Chapter 8 formula gave a CG position at 18% of mean chord, which gave satisfactory handling.

If one wing is swept as on a Pitts, or both are swept like a Tiger Moth, remember to draw on the side view the MEAN Chord of each wing, not its root chord.

Alasdair