A bit of a PSA (public service announcement): if you are installing a high end graphics card that has peak power usage of MORE THAN 250w (watts), you should run multiple, separate cables to it from your PSU. Otherwise you could be underpowering – and harming – your expensive graphics card.
If you’ve got a high-end graphics card that has multiple power ports instead of just one (you need to actually run two power cables) then what in general you shouldn’t do is get a single power cable which… with a pig-tail connection, and then you should NOT in general use both of those to plug in.
The reason for that is that a single cable like this usually has an output of around 250 watts, so if your graphics card – if you’re going to be overclocking it or regularly running it at max – or your graphics card simply requires more than 250 watts – then if you simply use the same pig-tailed connection, then that can be quite bad.
It can lead to crashes, and in the worst case it could actually damage your graphics card. So in that case what you should do is… that you run two separate cables into your power supply unit, and then you should literally just use one of these two cables each.
So basically you’d come along and just use that one, and then with the other cable you’d come along and just use that one.
There! And that’s the safest way of doing it.