Use the 2 separate cables. The one cable that has both connectors on it is only so that it can be used with EITHER one, NOT both. If you try to use just one cable with both connectors, it will not work.
is there any harm will happen to the card if I connect one cable in both ?
Probably won't hurt the card, but it may not turn on as it will not be able to get enough power through just one cable.