I fully agree with Jester, except that the location of the total CG also depends on the area of each wing and the type of stagger.

The CG is located at the middle point only if both areas are equal and if the stagger is negative.

If not, the CG will be closer to the CG of the higher and bigger wing.

Note that the point which is determined is the Aerodynamic Center (AC) on the Mean Aerodynamic Chord (MAC) estimated for the two wings working as one (disregarding the effect of the AC of the horizontal tail).

Note that the AC is expected to be located more toward the leading edge of the upper wing, because the lower wing lift is expected to be reduced (about 65%, depending on gap height and stagger) by the washout produced by the upper wing.

http://www.geistware.com/rcmodeling/cg_super_calc.htm