The bottom one has holes in the sides near the bottom. It just keeps the upper ones above the sump water level. The 2nd filter has the scrubby pads or bio balls in it. The idea of a wet-dry filter is that it needs to be wet, but not submerges. The bacteria needs lots of air to do their job. The top filter is a bead filter and removes the particulates. Particulates are bad for the bio-filter and clogs it up.

Only the top filter needs to be periodically cleaned as it plugs up. It will start to overflow. It can be churned a little, just the top inch of beads, to extend it another day or so.

I'll see if I can take some pictures of the entire setup.