I don't remember if I mentioned, but I did try to make a double-jointed knee. This proved to be an exercize in futility, frustration and madness, and I'm not sure I'll going to attempt another one, at least not until I've gained some serious wood carving skills.
Basically, making the spheres even is super difficult - and if they (or the sockets) are asymmetrical, the joint will just stick in certain positions and be impossible to pose in any position inbetween. Super frustrating. See video for example:
In the end I decided it wasn't worth driving myself crazy over. I haven't decided how to proceed yet, but I'll probably either go with single, Fashionista style knee joints for now, or try using wooden beads like suggested.
I'm also thinking of carving out the hip sockets some more so the doll won't look so weird while sitting. Hopefully there's enough material for it, and it would also make the torso more proportional (right now it's a bit too exaggerated even for the bjd esthetic).
If there's one thing I learned in (though not necessarily from) art school, it's that you shouldn't have too much respect for your sketches and studies. Overwork them, ruin them - that's how you learn. Force yourself to not Ctrl+Z at the slightest mistake. It's something I keep having to relearn, particularly after periods of low artistic output, but it's something you absolutely have to to keep in mind in order to get anything done. So I may have to ruin this experiment, we'll see. But that's alright. I found a nearby place that sells wood. I'll just start over again.