Gallons, Friction loss, size question.....

[whatcaneyedo]

These are high flow - low pressure pumps so I think you're going to have a pretty big drop in flow with this configuration. I wouldn't do two darts as return, leave one as your return pump and set the other up in a closed loop system.

This head loss calculator should be able to give you a good estimate on how much flow you will achieve http://reefcentral.com/index.php/head-loss-calculator.