Update Of The Day:
"Christophe" on the MD site has an idea which could greatly improve a screen's performance after cleaning. As you know, after cleaning there is very little algae remaining to do any filtering. One way around this has been to clean half the screen each week, and another is to use two screens, cleaning only one screen per week. Yet another way is to pancake two screens together which makes extra deep holes for the algae to grab on to. (By the way, "rug canvas" holds on to algae much better than "plastic canvas", it's just more flimsy and hard to work with.)
Christophe's idea was to use Lego Base Plates (the ones you played with as a kid):
They are available all over the web, and at almost every toy store and discount store. The beauty of these plates is that no matter how hard you clean/scrape, algae will still remain in-between the pegs (except the first week or so, where it will all come off anyways). Of course, you'll still want to sand/scratch all the areas in-between the pegs, but overall this looks very promising, if someone else would like to try it.
One disadvantage is that the plates are not (at least that I could find) available in clear, so a light on one side does not benefit the other side like it does with a screen. But since these plates are only formed one-sided anyways, it might not be such a bad thing, and indeed would be perfect for a twin-screen one-bulb setup.