I see people talking about trolls having the same last name if you have the same sign, but like the twins have the same last name and they don't have the same sign

That is true, however we do not know anything about the Twins thus far, because they’re an extreme oddity in the system. Once we know more about what it means for them to be Twins we’ll see more about that.

Besides their Signs are listed as two different Aspects on the Extended Zodiac but, let’s be fair, it’s just the same sign flipped turnways, and Sprite Flipping is just A Thing that happens so they maaaay as well be the same sign?