The spitter cable I linked attaches to where the ORIGINAL 3.5 drive power came from and provides multiple outputs from there. take power off the 3.5 drive and attach to the splitter   StarTech SATA Power Splitter PYO4SATA [URL] [IMAGE]  
I've just received my new XPS 8940. Before I fired it up, I wanted to install a the 3.5 hard drive and an SSD drive from my old computer. The slot for the SSD drive is at the top, back of the chassis and includes cables to connect it. No problem. The slot for the 3.5 inch drive is at the top, front of the chassis, just below the optical drive. I fit my hard drive in there no problem, but there are no cables to plug it in! I have a data cable and there is a free SATA connector on the motherboard. So far so good. But there is no available power cable. And if I go out and purchase a power cable, there is no place to connect it on the motherboard. There is a single SATA power connector, and it is already fully used by the other drives. To my amazement, technical support could not provide a solution, even after spending 45 minutes explaining what the problem is. Surely, I'm not the only person to deal with this?!?!
XPS 8940, problems installing a second 3.5 hard drive