I realize that the main advertising point for filters is flow. Most filters are rated for a certain tank size simply by their gallons per hour rating, assuming that more gph means a better filter. That isn't really true. Flow just means you are pumping water around. It doesn't say much about how that water is filtered.
Media capacity on the other hand says a lot about how much bacteria your filter can house, and how long you can go between filter cleanings. For canister filters 6L of media capacity means you can go twice as long between filter cleanings than if the filter had 3L of media capacity. That's probably not quite true for HOB filters, because there is a lot more bypass when the filter is half full with debris. Anyhow, longer intervals between filter cleanings are not only more convenient, they also mean you are disturbing your nitrifying bacteria less.
With all that said, having two sponges in your HOB almost certainly has more benefits than you would get from the increased flow by removing one sponge. HOBs typically have huge flow rates, while their media capacity is tiny. You are lucky if you get half a liter, while a medium size canister filter typically has ten times as much! So especially in a HOB you want to use all of the media capacity you have available!
