Hi Tarasco, good question. The two outputs on the closed loop are so incase somthing clogs one up the other can make up for it till I get to it. Like a snail or fish, even an anemone. It also reduces the suction incase a fish dose get sucked onto it.( eww what a mess that would be
) Not that I see any of my fish having that problem since there all large and there will be strainers on them.