The twin-core priori incantatum that happened between Harry and Voldemort at the end of Goblet of Fire was supposed to be a rare phenomenon (I don't remember where exactly this fact was mentioned, it could be somewhere at the end of GoF after Harry returns to Hogwarts, but I distinctly remember it being mentioned). But why is it such a rare phenomenon?
There are a limited number of wood and core types that can be used in a wand. If you take unicorn hair as an example, I can't believe each and every unicorn tail hair wand got its hair from a different unicorn. Unicorns are supposed to be rare, and on top of that extremely difficult to catch. Just in Ollivander's shop there are stocks of hundreds and hundreds of wands. I can imagine Ollivander grabbing a bunch of hair from a unicorn's tail and running away before it can stab him, and him making as many wands as he can out of them.
Given the small number of magical creatures available that can give up a body part for a wand core, I would think there would be a number of wands that share a core from the same animal. The priori incantatum that happened between Harry and Voldemort wasn't just because they were both phoenix feather cores, it was because the same creature gave the core for both of them, as explained in a number of places throughout the series.
So why is twin-core priori incantatum so rare?