This goes out for newbies or people who don't always read descriptions (such as myself): the cable either combines two mono signals into a stereo one or splits the thereo into two mono signals. Therefore, you can't "double" a mono signal as it were (let's say you're imagining you can take the signal from a guitar and connect it both to an amp and to an audio interface - not going to happen!).
That being said, it's still extremely useful for all manners of applications - for example I successfully split the stereo headphone output into two signals for L/R monitors (yes, not the most orthodox studio engineering, but it worked for me!). It's awesome that this piece of equipment is cheap, yet seems sturdy enough.