The aspect ratio of multi-wing airplanes is normally calculated as the average aspect ratio of the individual wings, weighting the average in relation to the area of each wing. In reality, the effective aspect ratio of a multi-wing airplane will normally be much lower than the geometric aspect ratio, due to mutual wing interference. At a normal gap/chord ratio, for example, a bipe with a geometric aspect ratio of 6 will exhibit induced drag typical of a monoplane with an aspect ratio of about 3.5. Increasing the gap will help raise the effective aspect ratio. This is often called the Monoplane Equivalent Aspect Ratio.

I like to use higher than average aspect ratios on my aerobatic bipes, in order to reduce induced drag. Normally use about 8.0, and have gone to 10.0, with good results - nice flat glide, much less speed loss in tight turns.