I doubt there are any threads dealing completely with this topic, but I could be wrong... Others, I believe, just deal with uses of magic, not possible backlashes. Or a little on the backlashes. *shrug*
Anyway, perhaps less powerful spells could backfire on the caster. Such as minor explosions that harm the caster, or a sudden draining of a stat.
Your post reminds me of a scene in the WoT series. It is when a \"bubble of evil\" surfaces and one of the main characters, surrounded by mirrors, has his reflection (in all the mirrors) crawl out and attack him. The former reflections drain the character of his energy. Rather similar to having a soul break into two parts... Except with different causes. Perhaps your otherself could be either hostile or friendly, depending on your luck.
Is it really a bad thing if you divide your soul into two parts? It would be if your NPC persona was hostile, but if they were another companion, it would be only an addition.Something that actually makes soul division a negative thing.