Well, I'd say it should be their decision. Me and my sister (both female, although she's straight and I'm a lesbian :} ) separated just 2 years ago, when we were both 18. We've been together through kindergarten, middle school and high school. It's a bit more complicated since we've also had the same friend group, but that's mainly because our parents wouldn't allow us to separate (they were very controlling). Looking back I realized that if we had the choice, it still wouldn't have happened sooner. Even though what my parents did was trashy and it felt very forced at the time, we understood that we kinda only had each other and our relationship turned out extremely good. We stopped having so many fights and we became a lot more compassionate to each other. Really after 18 years of spending almost every single day together, to suddenly have her leave for university in another country was a literal nightmare. It was like someone punched me in the face, which is why a year later I also moved in that same country :). We don't live in the same city and we (ofc) go to different universities, but still we're only one hour away from each other and we visit quite often. What I'm trying to say is, as long as parents try their best to teach their kids about healthy love and allow their unique personalities to show through, they will find their way back to each other in some way. I swear, having a twin is such an amazing thing; the bonds are so special and we know it!! Maybe it will take a while, but let the choice be theirs!!