Techea

If you get the same temperatures when you mount them both in parallel and series configuration that means there is something very wrong with the design of the gpu block. 

Go and tell those suckers who saw faster booting time and swear on it. Those are the kind of people who can swear they saw Jesus at one point in their life.

Replace it with Colgate.
 
Belly laugh today.

First plug the EPS cable 8pin to the cpu header and also add the ATX 4pin to it because it’s a 12pin header

Ok, I see what you mean. I thought it’s a 12 pin header. Just plug EPS and ATX and don’t bother with the 6 pin. 

Don’t overclock the cpu until you get the 6 pin connector. 

Use a Pcie cable to charge the 6pin cpu header. They split (6+2) so you should be good. Try one cold I’m curious if they fit that header.  IDE Molex to 6 pin if nothing works. 

No water cooling system goes that long. The solution is to replace it. 

Inside should be a pump fan that circulates the water through the system. If that block got noisy the pump is gone. How old is the system?

Replace it with Colgate.
 
Belly laugh today.

Super fine.

Try another gpu if you have a spare. Most likely gpus do this kind of things when they get hot.