This question is more of a theory thing then anything but I hope to implement it into my build given time and money can be worked out for it. If money were no option, Ideally I would like to do an SLI setup and CPU that are all water cooled. I've just been thinking about it and wondered if placement in series of the loop is a noticeable difference in heat dissipation. So I'm wondering if anyone has experience or a video/article regarding this. This is all theory and not so much specifics in regards to any one MB/CPU/GPU combination as there are endless possibilities. 
 
                                       (or reverse order of GPU to 360mm  last and CPU to 240mm first)
Loop circulation 1: pump/reservoir > GPU > GPU > Rad 360mm > CPU > Rad 240mm > pump/reservoir
 
                                                  (or CPU > GPU > GPU >)
Loop circulation 2:  pump/reservoir > GPU > GPU > CPU > Rad 360mm > Rad 240mm > pump/reservoir
 
                                                                               (or GPU > GPU > CPU >)
Loop circulation 3: pump/reservoir > 240mm > 360mm > CPU > GPU > GPU > pump/reservoir
 

   I put these in what I think would be the best order, 1 being best and 3 being worst. I would like to assume that loop 1 would be the best(?) option for keeping everything cool. Since the GPU's would be more then likely hotter then the CPU. So those 2 would get the 360mm rad and the CPU would get the 240mm rad. Also being as then any heat source (GPU or CPU) goes from the heat source to a radiator. Not accumulating all the heat from GPU > GPU > CPU (or vice versa) then to the rads then the pump. Finally the 3rd loop being worst just because the hot coolant from all 3 of them would then be going through the pump before they get to a rad to cool down, possibly shortening the life of the pump?

   I'm really interested in what anyone has to say and if they have experience with this or know of any videos/articles pertaining to this topic. Thank you!

Thanks for the reply ty_ger07, I have done a little more googling and came across this EK article on the topic. 

https://www.ekwb.com/blog/does-loop-order-matter/

Pretty much says the same as what you mentioned but is more for keeping pressure up in the loop. I think this sums it up pretty much from it.

"Radiators are usually placed just before the reservoir in order to remove heat before the pump, but you can also have radiators between arrays of blocks (largest radiator before GPU and smallest radiator before CPU for optimal thermal performance if your chassis can accommodate it)."

I appreciate the help just wanted to make sure it wasn't some sort of unwritten rule or I was missing something. Thanks!

Thanks for all the info Cool, I'll do some reading then. I'm sure it is explained somewhere with all of those articles but quick question about parallel and serial for GPU's. Series is the typical scenario for a CPU and SLI setup, correct? Parallel seems to be only for tri to quad set-ups or if there's more components like MB blocks and RAM blocks to reduce resistance?

Thanks cool yah that was the article I posted the other day after finding it. I appreciate it!