The RP-452X2 ships with one pump "blocked", so you can use it that way just fine.
If you connect "P1 [IN]" and "P2 [OUT]" with the rear pipe connector on the bottom, then use the internal acrylic bypass inside the lower reservoir, you are running into the top reservoir first (blue path in diagram below), then into pump-2, through the rear connector, through the front acrylic piece, and lastly in/out of pump-1. However, if you place the pipe connector on top ("P1 [OUT]" and "P2 [IN]"), then you're running through that reservoir
in between the pumps, so you would lose some performance.
http://www.koolance.com/water-coolin...p-452x2_d5.jpg
Tim