@Op -
You'll need to plug in both molex connectors - if you don't, you won't have enough juice going to the 6pin.
Normally, a molex has pin 1 suppling 12V, pin 4 is 5V and 2 and 3 are ground. 6pin pci-e has pins 1-3 suppling 12V and 4-6 are ground.
Its hard to tell from a picture, but the first duel molex to 6pin adaptor looks like its routing the 12V and ground from each to the 6pin (you don't need the 5V for the GPU card). You don't get all 3 12V lines since the molex can only supply two - but its probably enough for your card (check to see what kind of 12V amperage you need).
As for the rails - its a nice bonus if you can get the molex on seperate rails if you PSU supports that. But even if on one rail, you need both connectors so that you can get both 12V lines from each molex to the 6pin pci-e.
Later,
Sdrac
@delluser1: the later connector you showed only had one molex. I'd be careful trying to use that one as you could underpower the GPU.