In this world, the soul you're bound to has no way to hurt you. They cannot harm you, by purpose, by mistake; this is law.
In the middle of a war, two bullets glance harmlessly off the two soldiers. No skin was broken, no blood was shed, and now, no bind could possibly be allowed take place between the two.
What will happen when one is very calm and peaceful sweet sunlight and another is like dark and terrifying darkness of the night. When such completely opposite images will clash with each other. The sweet sunlight will be scorching or the darkness will be bright, it will be a sight worth seeing. Or a completely different world will be created which neither of these two would have wanted nor ever thought of.