If you fill in the gap and think of it as one surface then the aero center will be at 25% of the local chord. For stability you put the CG in front of that.

Think of it as two separate wing surfaces. The forward wing will have a aero center at its 25% chord and the aft wing have its aero center at its 25% chord. If the wings are the same size then the net aero center is basically half way between the two wings. The CG will need to be in front of that.

Based on the relative areas of each wing the aero center will shift forward or aft a little.

There is a lot of difference in the CG for the two approaches.