I have two as well and they work great.
For the carbon one, if you're using larger pellet carbon like kent, you won't need the sponge on the bottom, I just use the one on top as I run them full out and don't want any rogue carbon pellets escaping. I find less sponges cut down on the detritus that gets trapped in there. I do use both sponges on the phosban one as it catches a lot of phosban bits that try to escape when I'm first flushing the phosban.