The First King and Queen began the tradition. It was intended to end a war that had made each of them widows. The plan was brilliant, in its way: they would marry each other, unifying each of their peoples under one standard. They would each maintain their beliefs; he believed in the path of the Left, and her of the Right, though it may have been the opposite. The First King and Queen dissolved existing marriages and made their fellow princes and princesses find a mate of the opposite sect.
And as the royal family did, so did their counsellors, advisers, and the gentry, and the merchants. Eventually even thieves and whores took new spouses.
Those who did not adhere were tied to four strong horses and drawn in the cardinal directions, regardless of their previous allegiances. This made the King and Queen very unpopular for a time, but with the war still a recent memory--many of the warriors had scars and missing limbs and terrible burns--the return to war seemed much worse than a new wife or husband. In fact, many men secretly (or not so) relished the idea of a new wife (and more than a few women for new husbands), and many were thereby eventually tending towards happy.
And so the tradition of inter-sect marriage was born.
To maintain the balance, the eldest of each family's children would, at the age of nineteen, choose his or her direction: Left or Right (or the other way around). This would, thereby, dictate the religion of the other child by alternate paths. This would ensure that neither belief would enjoy the true majority. Both young men and women were allowed to choose, as long as they were eldest, as the First King and First Queen found the genders to be equal in such decisions.
And so it went. For generations. And as traditions do, it became ensconced in law for no other reason other than it was the way it always was, even though that is not how it always was. But only the historians knew that. One day, by decree of the Council of Orthodoxy, all other beliefs were punishable by death, and soon thereafter even those other beliefs became unbelievable.
After 15 generations, the eldest son of the new King and Queen came of age and he would not choose his direction. By law, he should have been put to death, but as this was the eldest royal male, he would inherit the throne, and regicide was not something the Council on Orthodoxy wanted to have on their hands. It was deemed a family matter and the King and Queen were quietly encouraged to persuade their child to come to a conclusion. And the young prince's birth records were altered to give them time to make him change his mind. But he never did.
Instead, he started speaking about a synthesis of Right and Left. He called it Up. He said that there were many noble traditions from both Right and Left, but he believed in both and that both could be embodied in the same person. He did not encourage others to join him, but he simply stated his belief, if asked. Often such heresies were humored when spoken by youth, but when it came time of choosing, it was time to put aside childish things. But he spoke his mind and, when interrogated, answered questions. And he answered his interrogators deftly.
After months of being ignored, The Council on Orthodoxy quietly convened subcommittees on Actions of the Right and another on the Actions of the Left. The Right concluded that the prince's voice must be silenced. They sent thugs to cut out his tongue. When the thugs returned with the voice of the prince, their reward was not the gold that the subcommittee committed, but the thugs were arrested and were summarily put to death on charges of attempted regicide. The prince sank into a depression at the loss of his voice and he spent many months in his chambers, not seeing anyone but the servant who brought and masticated the food for him.
But then the prince then started writing.
This went on for some time, and the the subcommittee of the Actions of the Left concluded that the prince's writings must stopped. So they sent thugs to cut off the prince's hands and to burn any writings. The actions what of happened before, when the thugs returned with the hands on the prince were echoed, and the amputating thugs were arrested and executed without delay. As before, the prince went further into his depression, but now additional servants were required to assist him in the smallest tasks.
And so the prince resolved to kill himself, but told no one, for he had no mouth to scream.
The King and Queen were grieving at the loss of their son to such thugs. Two assassination attempts were unheard of! And with such symbolic brutality! Their informants from the Council on Orthodoxy were silent in between the lines of their letters of sympathy. They felt they must maintain the strict separation between the Right and the Left and it is only through the respect of the royal family's position that the prince was not drawn and quartered, as per custom for heretics.
The prince was surrounded by servants. He felt obligated to them. When he performed the deed, however when it was to come, they must all be beyond reproach. It was in this way that he delayed his own death. He worried about the loss of their livelihood. For a time he kept up living and endured, merely so that they could feed their own families. He could confide in no-one the precise words, and the mere gestures of his arms made him feel stupid and incomplete. He no longer thought of Right and Left or even Up, but rather Down.
But that was how he decided to die, you see.
When the time came for his servants to change shifts at night, he rose and awkwardly, with his toes, he opened the latch on the window. Then he climbed out onto the balcony. His stumps hurt, as if his hands were still there, making fists. His soul ached. He wished to cry out, but all he could utter was a hollow groan. Release was soon, though. He climbed up onto the balcony and looked down. Below was a courtyard of stones. He climbed up and jumped off before any of the servants would come stop him.
And so that is how the prince died.
The priests of the Right and the Left both eulogized him in their ceremonies. As was tradition, the "youth" of the prince, even if they knew it was false, was emphasized. Though he had not chosen a path, they both claimed him. They ignored his belief that he could live from both sides, that each side was as good as the other, and that he believed in both. They ignored that they had driven him to this. His blasphemy was ultimately inconsequential. In the records, it never happened. And the servants were executed for leaving him unattended for those two minutes.
A single servant escaped notice. She was not older than puberty, and she was the eldest of her parent's children. She had heard the prince's words before his tongue was cut out and read his writing before his hands were stumped and so his ideas lived on. At least until she was drawn and quartered as a heretic; she would not choose a path upon her ascension to adulthood at the age of nineteen. She was not royalty, after all. Thusly, the hope the prince and his follower had for the world was wiped out.
The next eldest child chose her path, following her mother. And so the tradition endured.