You can use the following to find the CG of any biplane or even monoplane for that matter. First treat both wings as one: ie if the wings are 6" chord and the top wing is 2" ahead of the bottom, then that's the same as a 8" chord. You then balance at around 30% chord depending on design and use of the plane.

The CG for a biplane is exactly the same as a monoplane. Just work out the 30% position for each wing individually and if the wings are the same size the correct CG position will be half way between the two. If the top wing is bigger which it sometimes is then the correct CG position will be closer to the top wing 30% position in proportion to the difference in areas.

For example say you work out the positions for each wing and 30% on the top wing is 50mm forward of the 30% position on the bottom wing. If the wings are the same size the CG should be in the middle i.e. 25mm from both. However if the top wing was say 60% of the total wing area and the bottom wing 40% you'd put the CG 20mm from the top wing 30% (and 30mm from the bottom wing 30%).