The 8-pin PCIe connector has 2 additional ground (neutral) pins vs the 6-pin. It does NOT have additional +12V wires. The additional grounds lower the total CONNECTOR resistance and allow more power transfer, but in reality a 8-pin does not have twice the capacity of a 6-pin (which the 150 watt vs. 75 watt rating would imply).
Two molex to one 8-pin is a better solution than 6-pin to 8-pin, since each molex can handle 2 ground circuits. The molex connector is rated for 8 amps per pin on standard 18 ga. wire, so 2 connectors (2x +12V and 4x ground, total) can handle more than 192 watts on the +12V and 384 watts on the ground circuits.
If the card has 2x 8-pin connectors, you MUST connect 2x 8-pin power sources. The PCIe slot and a...