I run two Eheim Pro 350's on my discus tank. If you already have satisfactory surface movement, which I'm going to assume you have given the returning outlet you already employ, I would encourage you to take the second return lower into the tank. My first external I returned using two spray bars joined to give me surface movement over 2/3 of length of the tank. Now, when speaking to a plnt fanatic friend over water movement in the tank he suggested "dead spots", or areas without any flow, should be kept to a minimum. For this reason I took the second return down into the tank, it's 24" deep, in a central position. Then using a tee piece, small prices off tubing and a couple of 45 degree elbows I split this and aimed it sideways and outwards to give better movement lower down. The 2 outlets sit 5" above the substrate.
Hope I've explained that in a way you can understand. I can see how well the distribution is as I also have my inline co2 diffuser running through that returned so I'm able to see the gas moving throughout the tank.