Nope, you wouldn't need two cables. That's why the flex cable folds around the GPU's ... foot? connector? thingy? in a "U" shape. It can then have contacts on both sides quite easily, as it would be the same side (outer/bottom) of the flex cable (and flex cables with surface contacts or multi-layer flex cables are definitely not a new invention). Still, a picture says more than a thousand 68 words:
But you're entirely right, we have no idea if this would work. Which is why I'm trying to figure out if some of the brilliant and resourceful people on this forum might have some feedback, or want to do something with this idea. I know I have neither the resources nor knowledge to do so.
Oh, of course, definitely with a PCB of some sort on the end. It just depends on what it would connect to. For m.2, the logical thing would be a tiny board with an m.2 slot and the smallest possible power connector (berg connector? ugh! but might be necessary). For a second GPU, it'd need an x16 or at least x8 slot, obviously, and it would need a PCIe 6-pin power connector powering the slot.
But a PLX chip shouldn't be necessary for use with motherboards that support PCIe bifurcation. I don't know (at all!) how this works, but at least with all the commercially available PCIe x16-to-any-number-of-m.2 risers, they don't have PLX chips on them (though they seem to have stuff to help with power delivery). That is how I've understood this, at least. Intel MSDT doesn't support bifurcation, so for them, these cards are limited to X299, but they should work on AMD boards. (Please correct me if I'm wrong here.)
Also, designing my own case won't help me add more m.2 to a board that only has one slot 
Besides, if this is possible, with the tiny size of m.2 drives (and a long enough flex cable) this could work in pretty much any case.
Wikipedia: PCIe Pinout. Besides the lack of even rudimentary googling, how much weight are you putting in a 1-minute pencil sketch from someone who freely admits he doesn't have the slightest clue what he's doing? Still, even I could figure out that there are 49 pins per side for the first x8 and 33 for the second x8.
Also: "schematics"? That thing there?
