Just FYI, I went ahead and put another biological sponge filter in the Duetto. I tried to put the image on Photobucket but couldn't log in for some reason so I created an Aquarium Advice photo album and put it there. (I finally returned the borrowed biological filter so there is a picture of my tank there too.)
I actually bought this filter a couple of months ago before I ever heard of the Duetto. I had it in the tank with my Penguin Biowheel. As you can see, it is not quite square. It probably would be if it were new. You can't see it, but there is a small hole in the top, in the center, because I had it attached to the Biowheel's intake tube. Regarding your concern that it would slow down the water flow, if anything it may have increased it compared to the floss that was there before. I figure I'll rinse this sponge off monthly in the discarded tank water so it doesn't clog up.