Absolutely not, do not try to split a PCI-e power cable into 2 parallel circuits, you will fry your PSU. Get a 4-pin molex to 6-pin PCI-e connector adaptor, those are safer to use. Just make sure you have nothing else attached to that string of molex wires coming from your PSU.
Your video card should come with one but if you have a good psu use a mole to pci
Thanks for the advice however I can't find a Molex cable on my PSU
Seasonic X650 on sale for $80 right now :)