I have not much confidence in cable converters. But in your case, I suggest to just use the molex pheripheral to 2x6 PCI-E connector since that is what came with the card. Thus, the first card will draw power from the 6PIN PCIe that comes with your PSU while the 2nd card will therefore draw the power from the MOLEX to 6PIN converter that came with your card. Seems the safest bet for me.
1. Two molex pheripheral to 2x6 PCI-E (this is what came with the card) Best option
2. A single 2x6 PCI-E cord that splits into two PCI-E on opposite end. (will this limit the power my card gets?Probably) Not a good option
3. A single 2x6 PCI-E cord and a single PCI-E 2x8 with a 2x6 adapter on it. (im guessing this is worst option) Will work, but check you 2x8, it may be molded to allow you to remove 2-pins, making it a 2x6 ..........................
Side bar question: People talk about splitting the load on the PSU between the rails. How do you tell which rail a certain device is on? You would have to use a PSU tester, but most PSUs with multiple rails do not have more than one 2x6 or 2x8 plug on the same rail.
If you need more info, please post your PSU make and model number.
I downloaded your PSU manual. You have 2 12V rails: 1 is 16-18 amps and the other is 17-18 amps. The PSU has two PCI-e connectors on the same power cable, so it is likely to be on the same rail. Try those connectors first on your 5850. If it doesn't work, use 2 of the molex connectors to the 6-pin cable that came with the card and one of the 6-pins on your PSU's modular cable. Of the options you listed in the original post, try option:
-- 2 first, then
-- 1, then
As long as you are not cross-firing 5850s, your PSU should work well with your rig.
I thought you said it was a bad idea to use the splitter?
I used the splitter and the card runs fine so far OC'd to max on CCC. I'm just thinking I could get more out of it if I had two moxel cords powering it off separate rails.
That was before I read your PSU manual, saw the PSU power label, and saw the PCI-e cable configuration in the illustrations. Many PSUs are marginally OK, and it seems to me that most people end up buying a low-cost, poorly performing PSU after spending a lot on the other components. You did not do that. If you are OCing and its functioning well, be happy.