I am in deep affirmation in this topic, due to the fact that when you look at both the Jedi and the Sith you see that they are just force users, and they are the same. Sure the sith focus on hatred, but they still use the force. And if you just look. Some sith happened to be jedi until they were kicked out of the order. But I can tell that one day the Jedi and the Sith will put aside what ever they have aginst each other and unite in one full united order.
I apologize, but this could never happen. If it did, what would they be fighting? It would just become a cheesy storyline like AdventureQuest Worlds. There is 1 reason for the split between the Jedi and Sith: Good vs. Evil. The Jedi wanted peace while the Sith wanted control. If Good and Evil joined, who would they fight? Chaos? Sadly, this seems like something Disney will do in the seventh Star Wars movie. Some Jedi and Sith can switch over, but nothing more.
1st of all Do NOT bring Disney in this they DIsgrace Star Wars. Anyway It could happen, heck I might even write a book about this topic, ANYWAY. They could unite if there is a time for it, If you have forgot the Sith were once Jedi that were kicked out for their ideals. And the Clone Wars the jedi did turn into warriors. But now I have a strong hope that both the Jedi and the Sith will unite