They exist, but they are a very bad idea. You'll be pulling double off that circuit than what it was specifically intended to handle. Again, fireworks, or if not, just problems in general. If you can't source the correct second cable for your empty PCI socket then getting a new PSU would be the better option. Don't use any of those adapters that allow you to pull 300w from a socket that was designed for only 150w. If that unit was designed to provide 300w from that single plug, it would have come with a cable already configured to do that.
It's a good way to ruin something, and will cost you a lot more in the long run than a new PSU would cost, or a proper cable.
Looking at reviews of that unit, it is likely that it is VERY old. That model has not been sold for a number of years and is probably around 8-11 years old. Not only is finding replacement cables for a unit that old likely to be very hard if not impossible, it's probably a bad idea anyhow. Any unit that old should be retired ESPECIALLY when we are talking about using it with a thousand dollar graphics card. Get a new unit. Don't mess around and take chances with your very expensive hardware.
Find something in the 750-850w range from among the models I've listed as recommended in my thread here:
Below you will find MY standard list of recommended power supplies and beyond that this thread is intended as a landing place for questions or discussions regarding specific units, platforms or related PSU tech, all of which are all welcome to be discussed here. If it's related to power supplies...
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