welcome to the forum. It's interesting to me how crossbows and other weapons are presented in video games, fiction, and movies. A lot of times it seems like the fantasy writers and game designers want to give their characters an assault rifle, but since that doesn't fit in with the setting they make a crossbow that is magazine fed and automatic. While such features may be possible to impelement, in practice the more complicated the weapon becomes the less effective it is. Kinda like the same reason we don't have flying cars. While you can make a vehicle that drives down the road and flies through the air, it wouldn't be as good at either of those activities as a single purpose machine.
I think the reason writers and artists want to present crossbows this way is, well, crossbows are admittedly cool, but drawing them and loading them isn't. I played a lot of Mount and Blade Warband, which is the closest thing to a medieval combat simulator that I've come across, and let me tell you, bending over and fiddling with you your weapon for even 5 seconds while enemies are charging and shooting at you is definitely not cool! Even in a "realistic" show like Walking Dead, I noticed that they don't show the dude actually spanning and loading his crossbow hardly at all, he just seems to always have it cocked and loaded and ready to go.
So, are such features as auto loading and a clever built in spanning device possible, in weapon powerful enough not to rely on poison darts and completely unarmored opponents? Probably, with enough ingenuity and effort. Would it be worth it? Perhaps as a work of art or a monument of engineering, but hardly practical. I mean, you saw how cool that twinbow is, and I don't think that company is still in business. Cool at it is, it can't compete with simpler, less expensive, more powerful crossbows, not to mention firearms.
That being said, one "fantasy weapon" I have been toying with for a while, and probably will build a version off inside a year or so, is the "double-barreled" crossbow from the movie "Ladyhawke." Not a fanboy replica, but a functional weapon based on that concept. I know it will be heavy, and complicated, but at least you can shoot it twice before reloading!