What Coinoisseur says...+ What mining software, OS are you running?
Manufacturers aren't going to add more copper to allow a wire to handle double if they don't feel it is a need. They design with the amount of copper for the job of the connection and it's power needs, no more. So, if you have exceeded the electrical requirements of the system with your setup, it will bug out, harm the devices, or harm the system.
I would just connect straight to the power supply one card at a time and test to see how each card is functioning. If the card runs, let it hash and play with the clocks to see how it responds.