Would they allow a person to use them both?
That's probably up to the runes themselves. We know the Night rune has a strong personality and even attacked Viktor for leaving him in the Cave of the Wind, and it jumped out of his scabbard when they had their tiff during Tessai's recruitment scene. The Sovereign rune didn't display much of an attitude (Barbarossa described the rune as if it was attached to his sword, unlike the Star Dragon Sword which is the Night Rune itself transformed into a sword so I don't know if that would make any difference)
But I don't see why someone who is tough enough to carry two big ass swords couldn't weild both. Whether the runes get along with each other or the person carrying them is another question.