This is a very interesting idea. I like the "twins are special, everyone else are outcasts" idea. Adding to that. What if, even thought twins are superior, only one is allowed to survive. Until a certain time in theri life both twins, maneuver and plaot and train for a predetermined date when the will face off and only one will survive. Sibling rivalry would be encouraged. Maybe a coming of age ritual is besting your twin. I could go a number of ways, very interesting indeed.
I also have an Idea of the Day I'd like to share, and I thought it would be appropriate to post it here.
Magic System- Magic users can travel to another world/ dimension where however much time they spend there only takes up one second in the real world. If a warrior was about to be defeated, he could travel to the other world, train for days, weeks, months, years even, and then return and better his opponent. Maybe this world is empty space, and whoever is there can shape waht they want it to be. That same warrior could make copies of his opponent ot train against. This warrior scenario is only one possibility, ther are many others, post your thoughts. A possible cost for this system could be that the more time you spend in the other world, the less connection you have to the real one. Extended periods in the other world could result in slight or complete loss of smell, touch, sight, or taste in the real world. Post what you think about htis idea or possiblities for it, or alternative costs for using this sort of magic.