You are talking about the preview bounds of the shape. Select each stroke in the Appearance panel and in the Stroke panel next to Align Stroke, click the appropriate button, hold mouse over each button to see the names. I guess you want to set the alignment for the inner stroke to Outside and the outer stroke to Inside. However this will shrink the preview boundaries of the shape with half the width of the current strokes. To compensate you may need to change the size of the shape and offset of the second stroke with the same amount. Then when you chage the with of the two strokes the shape will stay the same and only the 20pt gap will shrink.