My return simply returns water from the sump. I only have (maybe) 400gph, so regardless of how many lines I use, it's not adding to flow in any amount worth worrying about. The only time I used dual returns was on a 150g running a MAG 18, and even then the flow was minimal.
IMO, use the return pipe how it best fits and address flow using pumps designed for that.
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Brad
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