SC- Years and years later, they meet Helena and Xander. This is another part I'm not quite sure about yet though- they either become part of the royal guard before the plague, or they meet after the plague starts when Helena and Xander have to leave the castle. They'll probably be traveling together for awhile.
OLAS and forward- Skipping waaay into the future, the twins are still alive, living in an abandoned city in the desert. At some point they're going to meet and join some of the other characters. I'm thinking they might meet Vyra and Ace first. (and maybe some people from the group Ace was working for?)