The only way I know of that you can get away with it is if you modify a hang-on power filter to be a refugium. RockChuck is right, there is no way you want to mess with 2 pumps, it WILL result in overflows. He is right again about the above tank refugium. That plan requires that the fuge be drilled or have an overflow box. It doesn't need to be more than a few inches higher than your display tank, by the way. Back to my powerfilter idea. With this, you would take a powerfilter, remove all the insides and put a little substrate in it and then put a light on top of it and add some macro algae. This is only going to be effective on small tanks. Also, someone makes a hang-on refugium that does this exact thing, except is was specifically made to be a refugium, obviously. This will really only be effective on a small tank as well. IMO, if you have a tank that is bigger than 30 gallons, you will need more than a hang on refugium.