There is a post in the forum here made about the Z620 power supplies and the designation on the power supply label specifying how much wattage can go through each 6-wire PCIe supplemental power cable.... they can carry significantly more than the ATX standard of 75 W each. For high power video cards bought through HP for their workstations there are references to included splitters and converters that take a single 6-wire plug and convert it to two 6-wire plugs, and a single 6-wire plug and convert it to an 8-wire plug, respectively.
I'm sure the same applies to the Z840 power supplies.... they are built to standards well above ATX. I have no idea how to get those converters individually from HP, however.
The link to that post is HERE.
Note that there also is an added link in that post to HP documentation that mentions those adapters...... the HP power capacity for each of their 6-wire cables is much higher than you'd expect, as you'll see (216 watts each, from the Z620 sticker on its power supply).