This is one thing I don't know why more companies don't do.    Simple way to overengineer .... include a second power connector.   It is like a $0.20 part (probably less in high volume) and it adds maybe $0.02 to asembly cost.   Just attach both connector to the same power plane.  

When running close to the limit it is an easy way to add "insurance".  Given that consumers may be using unknown power supplies from unknown companies it is a way to spread out the load and if the pins in a connector are slightly loose (increased resistance) it may be enough to prevent a fire.   This is one reason why the PCIe standard is so conservative.   Yeah a 6 pin connector "can" handle 300W assuming a tight fit (hot often will a consumer push the connector in but it doesn't seat fully and results in increased resistance), the power supply company didn't cut corners, some knock off OEM didn't cheap out and use 20 AWG wiring instead of 18 AWG or use some chinese knock off just slightly out of spec crimp pins instead of ones by Molex or other major part supplies.  

In the real world lots of things can happen which are unpredictable.  A graphics card which pulls 150W from the PCIe connectors will normally use two 6 pin instead of 1 pin pin.  It is pulling 75W per connector for one which optimally should be able to handle 300W.  That overengineering is cheap insurance.  Say it adds $0.12 in cost to a $120 card ($199 retail).  If it reduces the warranty failure rate by 0.006% you broke even.