The brothers, Blake and Josh Stagstone, had always been close growing up. As children, they were partners in crime, getting into all sorts of mischief. That did not change much as the grew and became more adapt in the arcane arts. Their curiosity knew no bounds, and their keen intellects made their harmless pranks marvelous wonders. Of the two, Blake was always the more studious and always enjoyed reading magical theory, or the occasional fantasy book. Josh tended to be the life of a party and always could bring a smile to someone's face. When they put their heads together, their devious streak shown brightly. After every successful prank, their trademark symbol of a two-headed stag on a rune stone would drop to the floor for a few moments before dissolving into nothing.
Despite the shared enjoyment for a joke, both took school seriously, though Blake seemingly more so than his brother. School severed two purposes, to expand on their knowledge bases and to give them ideas for future endeavors. Given their curious nature and need to take risks with magical breakthroughs, the Blakes and Joshs across many of the universes were bound to have spells or other magical concoctions backfire in unbelievable ways.
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In the first option, Blake (25) and Josh (27) were working on their shared masters' project to complete their graduation. They had started the master's course together, Josh having waited for his brother to graduate from college. During their studies, they decided to complete a joint magical project, which their professors approved. Prior research showed that identical twins had the strongest magical results when cooperatively casting due to their undeniable magical connection, believed to be through sharing portions of their souls. The current theory was that non-twins could never achieve such powerful spells because their souls would never be as linked. Blake and Josh wished to prove this idea false and that all siblings, even non-blood related people, could have similarly powerful cooperative spells.
The project was simple in theory. Blake and Josh would link their souls together, a non-common practice now but was more widely used during the Renaissance era. This required extensive research into the magic of soul linking and then putting it into practice. As the months dragged on, they were successful in linking their souls closer together similarly to that of extremely close identical twins. The benefits of that alone were noticed immediately. Each had access to the other's well of magical power, and jointly they had far greater magical stamina. Akin to the twin telepathy widely discussed in urban myths, the brothers had limited communication through their minds and they could sense the other's pain, pleasure, or extreme emotions. Being able to sense each other's pleasure was the most interesting if the managed to coordinate having sex with their partner's at the same time.
Then came time for the final part of their project. They were going to attempt a cooperative casting feat that had not been seen since the time before the Roman Empire. Neither brother remembers much after they initiated the spell, only that they were successful. After that, things became hazy. They woke up hours later sharing a single body. The rare Dicephalic parapagus twin mages were said to have the most potent of spells, and now the brothers, non-twins, had that same power.
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In the second option, the brothers, around the ages of 13 and 14 (as Josh's next birthday was near), were on break from school and decided to meet up with their friends at the local mall. Magic for underage spell casters was regulated when not at school, and most spells were not allowed. However, some spells were still acceptable as the world had long-since known about magic. It was no secret that about 10% of the population had some level of magical power. Even mundane individuals bought magical objects, like enchanted brooms, to help make simple tasks such as sweeping the floor easier.
While spending time with their small group of friends and gazing at all the objects they wish they could purchase, the mall was attacked by a masked group of spell casters. The mall security was quickly dealt with, leaving the group free to begin slaughtering the citizens. Those that could, teleported away, but there were still so many people left behind, as target practice. Fireballs blew-up and curses struck people down. The brothers were hiding behind a fountain, holding each other tightly. Given the rise in terror attacks, the schools were mandated to teach both of their grades the art of teleporting, the youngest group yet to learn. However, those lessons had only just begun, and both knew they were not ready. Then, a tusked-mask spell caster spotted them. A dark-green orb, of some unknown spell or curse, appeared between his hands and then came rushing at them. Both Blake and Josh remember the spell hitting them, and both reacted on instinct. While still clutching each other tightly, they teleported. One of the first rules student learn about teleporting is that anything someone could feasible carry or lift is teleported with them unless they focus hard to not take them. This rule includes people. The second rule is that other spells can tamper with teleportation.
For what felt like hours, the brothers were trapped everywhere but also nowhere. Unbearably loud sounds came from all directions, flashes of light in all spectrums dazzled their eyes. But above it all, they heard the tireless screams of their brother. They were stuck somewhere in limbo between the two different locations each had thought of when teleporting. Finally, it all ended. The quiet was almost deafening. Blake and Josh awoke in a field, somewhere between the two separate locations, home and school, they had envisioned. However, they awoke irreversibly changed. Their bodies had completely fused together in the form of dicephalic parapagus twins. Their physical features had completely blended together making them appear identical, some mix of their two individual looks.