I haven’t thought about Arnav and Alev in quite some time. They would be identical twins if not for the magic of their souls differentiating them- Arnav taking a blue-dominated palette for his water magic, and Alev taking a red-orange appearance for his fire magic. They shared a peaceful childhood, but in their adolescence their region became more unstable, their town taken over by a lich- allowed to live largely peaceful lives, except said lich would periodically take people away to never be seen again. This would catch up to them eventually, and it came for Alev. Arnav followed undetected to try to save him but underestimated just how proficient the lich was with its rituals, and so Alev was killed, and his soul poured into a particular malachite. Arnav does manage to swipe the stone, though, and a good chunk of plot later winds up infusing Alev’s soul into himself, now sharing a body with his brother to allow him to continue living. This disrupts everything- daily life, function, magic, even down to their now-shared fur color, which begins as a marbled red-orange and blue and over time meshes together into a purple as they learn to cohabitate and wield their magic in unison.
This forms a taboo in a number of ways, but the one that stands out the most in universe is that one body is casting two elements of magic. Each soul can utilize one element. No more, but sometimes less. This is something that many over the ages have attempted to accomplish but none have managed. This noteable exception, however, was made possible by the divinity of Genesia after her reawakening. Hers is the domain of life, of moving souls into said new life, which in this case was used to move a soul into existing life. Had a standard hexen (group of six casters, one for each element) attempted this, it would have surely ended in failure and possibly disaster no matter how synchronized its members were.