The 40i uses the same filter pad as the 20i, so doesn't really add ANY biological filtration, it just has a more powerful motor to circulate the water more rapidly (170 gph vs 125 gph).
This isn't going to really help water quality much, if at all.
You would achieve more of an improvement by removing the 20i altogether, and substituting a HydroSponge IV (rated for tanks up to 80 gallons) which is approximately the same size. It has VASTLY greater biological capacity than the 20i (I know because I own both, and use the 20i for a turtle tank).
Here is a link to some information about the HydroSponge (and sponge filters in general):