What is the maximum power for an 8-pin PC PSU cable?
Single 6-pin connector allows 75W power delivery, and single 8-pin connector allows 150W power delivery.
Since you need to power two 150W connectors it can draw up to 300W.
If you have split a single 8-pin 150W connector into two 8-pin connectors, it means that there can be double the expected load, so there can be double the expected current in the wiring and voltage at the connectors can be less than expected. Wiring meant to power a single 150W load may not be enough to power a 300W load.
Since the wiring which comes out of the power supply is factory-made with two 8-pin connectors that are 6-pin compatible, there is no doubt that it can handle two 150W loads or 300W in total.