....and good question..

Narrowing it down to mystical union between two people is certainly a good way to try and figure out what this is all about. Of course anytime I hear the word "mystical", I either think of something occultic or out of Rome, but perhaps that's the best way to bring it into perspective.
I think that anytime someone has been a sexual partner in an ungodly manner, or in any involvement where there is a resulting negativity, there is sin and consequences. Depending on the degree of attachment, the situation could possibly open the doors to Satan, although as I said earlier, this doesn't mean that a demonic attack is always the result of sin in someone's life. But sin will definitely draw after it's own kind. In these situations, the person needs to pray to be forgiven for having taken part in the sinful activity and the accompanying emotions, whatever those may be that are of sin. Breaking the soul tie then, would be an act of obedience to Christ to leave behind the relevant activity and thoughts. I believe some might think this also implies a "mystical" union, but I don't see where that is made perfectly clear in the Scriptures. It seems logical though, that if we can form godly ties in the spirit, as with Christ or one's spouse, then we can also form ungodly ones.
To me breaking a soul tie is an act of repentance as well as seeking help and strength from God, and if the sinful activity opened the doors to satan, then taking care of that situation as well, in prayer.
I did a quick search on the topic and came up with this flowery (literally, he he) website, which describes what I think I rather describe as a spiritual, rather than Mystical union. Yeah, I know, cheap semantycs, but I really don't care much for the other word......

http://www.b4prayer.org/index10.html