There is a spiritual phenomenon called “obsession,” but it’s pretty rare. Usually people just have psychological obsessions of perfectly natural origin. It ain’t good mental health, but it ain’t demonic.
One person can certainly be an “occasion of sin” to another person, but so can chocolate. If you can’t control yourself very well, you avoid the occasion of sin (easier to do and very prudent) or learn to control yourself better around it (harder to do, though sometimes necessary). I have trouble not committing the sin of gluttony around chocolate, but it’s not because chocolate is tied to my soul.
Anyway, all human beings have free will. A demon can tempt you, trick you, or even possess your body, but I seriously doubt that anything can tie your soul to anybody else’s. If marriage can’t (“till death do us part”) and the good Lord won’t (Jesus’ parable about the rich man and the poor man), why would a demon be allowed to? Is your guardian angel supposed to be drunk or asleep while the demons are tying your soul? Of course not. It’s sad and pagan for Christians to think that demons could be so strong and God so weak, as to allow a breach of free will at the level of the soul.
If you are really all that interested in studying the theology of evil and demons (and it’s not probably something to get into unless you have to), there are reputable texts you can look up. You may have to read Latin, but it is out there. In theology textbooks in big libraries, not in occult shops or weird Christian bookstores.
The kind of stuff you’re hearing is not stuff you should trust. It’s relatively new, it’s coming from people who may or may not know anything about what they’re talking about, and it seems to be based on extremely dubious theology. (And there’s usually a fair helping of folk occult theory in it, too, although the people doing “deliverance” usually don’t know that.)
