The rule of thumb I've seen in the past is that you want to have a minimum of one chord of separation. At 1 1/2 chord's separation they have so little blanking effect on each other that you can ignore it.

Full sized aircraft have flown with separations quite a bit less than one chord but there are drag problems associated with that method. I would consider going down to 0.8 chord separation but no less. And only if you NEED to. One chord is safe.

You'll want to reduce the chord down to about 1/2 or 0.6 of what it is. The biplane configuration concentrates the area without requireing a large chord so you can reduce the chord and the separation without reducing the wing area.