I agree that you should pipe your pair of collectors in series. Water is lazy. If your supply line is in lower right then couple collectors together to create one large collector and take return to storage out the upper left. Remember that the higher efficiency you are looking for is to send the coolest water into the bottom of the collector. The higher delta T transfers more heat in less time than running water heated in 1st collector to a hot absorber platein the second collector.
Ray Pokorny
503-449-0527
