I don't think it does come up again, and I wondered about it, too. It didn't make sense, either, that having two souled vampires would make the universe spin off its axis. There could be a million souled vampires, but only one is going to fulfill the prophecy. Wesley's translation says, "the vampire with a soul," implying only one, but I think it was even said in the series that the correct one would only be known after the prophecy was fulfilled, not that there had to be only one in order for the prophecy to be set in motion.
The Senior Partners being able to patch it also seemed too easy. I was expecting that it would be revealed to be something that Lindsey did, but I don't think the problem was ever referred to again.
