"Well, there would be several reason I can think of where it would be impossible or, at least, ill advised. True Runes choose their bearers. They can also be fickle and it's not unheard of for one to move from one to another of it's own will. Those that don't have bearers can and will remain hidden until they so choose to reveal themselves." Riou pauses to think of how to continue.
"They also have purposes, goals, or desires that they seek to fulfill and will do so without consideration for things such as morality. Having two would place an incredible strain on the bearer and the Runes could... oppose one another. What's more, a weak willed bearer can, and will, end up as a mere slave or vessel for the Rune, and it will not be the person, but the Rune acting out it's own goals."