From the look of it, left and right channels have common ground, but still all the cables I've found so far are just two balanced cables glued together. Why no one created 5-wires cable and saved 20% of the cost?
Sound quality is probably the first reason. A balanced audio signal only works when the interference/noise is the same on both signals. After inverting the sound signal, the noises that were identical on both signals will only be canceled out. So if there is any interference on the signal, make it happen that they are the same on both cables. When using a shielded two core cable is more likely to have the same interference than when using a shielded four core cable.
Usability is another aspect in choice of cables. A shielded two core cable is more flexible, especially the dual braid copper wires. When using a shielded four core cable you'll lose flexibility, durability and even the chance of exposing one of the conductors from its shield when flexed (read: more stress inside the cable when flexing).
Price A shielded four core cable is in general twice as expensive as an shielded two core cable. When one core breaks down you need to replace the entire, more expensive, four core cable while using two cables you only need to replace one of the two.
I use 5-pin XLR ends on a canare starquad cable to carry a stereo signal from my MS microphone pair to my stereo mixer. No problems so far. From my research this seems to be a common method and is supposedly especially safe over short runs. My run is 10 feet and I use y splitters that use mini starquad at both ends.
Yes there is, and it's called multicore, which is a generalized name, and can have as few as 2 cores, e.g.: http://katalog.cordial.eu/catalog2,25,29,mw41.html