Soulbonding is one of those things where you have to separate the fundamental phenomenon from the frilly explanations that people like to dress it up in. At its very simplest, a soulbond is an imaginary friend. Sometimes the soulbonder is actively creating this character, sometimes the character's existance is tied into a writing process, and sometimes the character is just there. There is no reason why someone's imaginary friend or the voice in their head shouldn't be a fictional character.

But when someone says imaginary friend they start being afraid that people are going to think they're childish or disbelieve them. That's where the ideas of alternate dimensions and different realities and astral travelling comes in. It's a way for people to convince themselves and others that the character is Real. We've seen people who couldn't accept the voice in their head as long as there was any possibility that the voice could be part of their imagination or originated from their psyche but as soon as they latched onto the idea of the voice being great awesome character who walked in from another universe then everything was fine and dandy. Then the soulbonding community met the multiple community and found a whole new type of real that they immediately wanted to attach to their soulbonds as well. (Funny how you find a lot of soulbonders who claim their soulbonds are just as real as the people in multiple systems but you never see them claiming that the soulbonds are just as real as their neighbor Bob who lives across the street.)