Wanted to share what happened when I used a dual 4-pin molex to a single 8-pin PCI-E VGA power adapter with a Sapphire Nitro+ RX 570 dual mining ETH that also had the 6-pin plugged in from a different PSU cable. The PCI-E 8-pin connector is rated for up to 150 W and clearly the 18 AWG copper wire wasn't able to handle the constant draw despite being split over two molex connectors.
A heads up to those who use 18 AWG wire in adapters and spitters to power 8-pin PCI-E VGA power connectors. If you're PSU doesn't have enough 8-pin connectors for all your cards, you NEED to get a bigger PSU or use a dual PSU setup so that you have enough connectors fto run all your cards safely.