But more difficult, yes? Stacking filters is very different from getting a thicker filter. The former increases air resistance, the latter decreases it. So t

But more difficult, yes? Stacking filters is very different from getting a thicker filter. The former increases air resistance, the latter decreases it. So to get the equivalent behavior from two 20x20 filters, you'd have to device some way of routing air through both of them in parallel.