A common cable included in modern modular PSU's is the 8-pin to 2x 6+2 pin. However, based on reading (such as [PCI-e Power - Does 6 + 2 Equal 8?](https://superuser.com/questions/992085/pci-e-power-does-6-2-equal-8/992090)) an 8-pin is effectively the same as a 6+2 pin. If that's the case, then how can one 8-pin connector effectively supply *two* 6+2 pin connectors?

My research so far has informed me that an 8 pin provides about 150W of power. One rumor has it that the upcoming 3000-series cards from Nvidia might utilize 3x 8-pin connectors. In that scenario, can you safely use **two** 8-pin to 2x 6+2 pin cables to connect the GPU (note: this would leave one of the 6+2 pins unplugged). Ignoring the power from the PCI slot itself, this would **either** provide 300W of power (for the 2 8-pin connectors facing the PSU) *or* 450W of power (for the 3 6+2-pin connectors plugged into the card).

On the surface, it certainly appears as though it would only provide 300W of power, but I can't understand why PSU makers would give you an 8-pin to 2x 6+2-pin connector if it meant that each of the 6+2 pins could only provide *half* of the power as a normal 8-pin (because, again, they're supposed to be functionally equivalent).
I've been trying to understand PSU cable specifications but have been unable to find a definitive source.