It seems that there is now a new model of PCI Express extenders for connecting GPUs to mining motherboards available in white color PCBs and with three different power connectors available. The new extenders come equipped with a SATA power, 4-pin Molex power and 6-pin PCI-E power connectors and you can apparently are able to connect power through any or on all of them. This would allow for better flexibility, especially in cases where you do not have enough power connectors of a certain type, but can also help in overcoming issues with too much power draw over a single power cable. Normally the SATA power is the lowest rated in terms of power capability and thus it is most often the cause of possible issues with burnt or melted cables with GPUs that draw more power from the PCI-E slot. Connecting a Molex to SATA power along with the SATA power onboard on these extenders could help in preventing such problems for example.
Regardless of the power connector options available on these and how you mix and match them you should still be careful how much power is being drawn over a single physical cable going to the power supply. More than two PCI-E extenders on a single physical cable is not recommended, though it also depends what is the power draw of the GPUs as some may have very little or even no power draw from the PCI-E slot, while others may have huge draw even if the GPUs are not with very high TDP. Even if the power cables themselves (18 AWG or even 16 AWG) are capable of handling the higher load the modular power connectors that connect the cables to the power supply are often the failure points as they are really not rated for that much power. So always be careful if your mining GPU rigs in order to avoid any possible hardware failures that can result in extended downtime to diagnose and/or fix afterwards.