Ooooh, now I get it! (I'm slow sometimes)
Well, a 1" bulkhead will have a flwo through of about 600 gph (if memory serves me), so 2x1" should be 1200 gph-ish. So maybe the mag12 is the better pump for the return, unless you capped on of the bulkheads. Regardless, your new design should be an...