I wrote up a really long explanation with RHUs(Random Heat Units) and RVU(Random Volume Units)s but realized it was way to complicated to make much sence so I found a
power curve chart. Lets imagine for a second the Maze 4 equals a foot of head. So figuring everything else in your loop equals 1.5 feet of head and you have an Eheim1048.
1 Maze4 115GPH Total Loss 2.5ft
2 Maze4s Ser. 85GPH Total Loss 3.5ft
2 Maze4s Par. 125GPH Total Loss 2.0ft
62.5GPH per loop
Cut the flow for the parallel system in half and you get 62.5GPH through each loop. A difference of 22.5 from series. Now I choose 1 ft of loss for the Maze4 at random. It would be a close call at this point what would do better parallel or series. Lets take this further through.
Lets say I decide I need a bigger radiator and now my base head loss is 2.0ft instead of my previous 1.5ft. So my new numbers are.
1 Maze4 100GPH Total Loss 3.0ft
2 Maze4s Ser. 62GPH Total Loss 4.0ft
2 Maze4s Par. 115GPH Total Loss 2.5ft
57.5GPH per loop
Now that the curve is flattening out we see a huge drop in flow for a highly restrictive system. The difference is a mere 4.5GPH. A little more restriction and the parallel may actually get more flow. Given the great increase in flow with parallel I choose it simply because now I have 53 more gallons per hour to cool other parts of my computer.
I've figured out 2 things in the past 5 minuts. Its crucial to check the curves for figuring out the layout of a system. And it would be freakin awesome if places posted numbers on headloss through their blocks.