Boy, sometimes a picture really is worth a thousand words. I was confused why you were saying that you turned one panel upside down. But having seen your set-up that makes sense, because you have your panels are laid out end to end, for a 4x40 layout, with the plumbing coming in the the middle where the two panels meet. In my setup the two panels will be laid side by side, such that the long sides will be adjacent to each other. An 8x20 setup. The plumbing will all come from one end, and the water coming in to the panels will feed into the bottom of each panel. Thus, there is no need to turn one upside down. In fact, turning one upside down would be counter-productive. Anyway, now that I have figured out your layout, I see that the plan I had in mind is in fact a parallel system.
Thanks for the photos.