The manual for the kryographics Pascal for NVIDIA TITAN X states in step 5, "Be sure not to connect any water blocks in parallel but only consecutively".
To that end, it would be useful if aqaucomputer provided a kryoconnect for these new blocks that could be made to go serial.

You could take an existing kryoconnect bridge, bore a hole in the side, at the center, to provide a passage between the two longitudinal bores.
Then seal the outside wall at the center bore, and included one additional headless set screw to direct flow internally.

I guess that's where the "personal" in "personal computer" comes into play

If you have sufficient flow that cutting it in half across two GPUs is acceptable, then yes, parallel is fine.
And I agree that parallel can be more aesthetically pleasing by allowing the IN & OUT connections to be on the same side of the same board, if that works for the loop.

My preference for serial stems from the fact that if one of the GPU blocks develops a minor blockage, the flow rate across that block will diminish, and the other block's will increase.