Help! What is the word for…

I am really stuck; there is a word, or maybe a phrase, which I once came across in an article and has now escaped me. I would be most grateful if anyone could help me find it!

It is a psychological term or phrase to describe a couple or pair of people; they may be siblings, or partners, or husband and wife, friends or strangers, but two people who are bound together by some crime or heinous act they have committed, and who go on to commit more such, egged on and encouraged by the other . Such pairs of people are well known, and I guess there are examples in every country and culture; in Britain for example one could think of husband and wife Fred and Rosemary West, the Kray twins, and school friends John Venables and Robert Thompson.

The words I am seeking are not just about the murder/torture/crime etc; there is a phrase which suggests that if the couple had not come together, their crimes may not have been so dastardly or may not have been committed at all. It is something about the relationship which conjured the evil they went on to commit. So Fred West may have raped and murdered someone, Rosemary West may have killed her own child, but together they created this monstrous unit which committed hideous acts of cruelty to an unknown number of women, including their own daughters. Without each other, the Krays may have been criminals and gangsters, but it was together that they became the most feared villains in London and beyond in the 1950’s and 60’s. Venables and Thompson, sad young boys from difficult and neglectful backgrounds were in many ways pathetic little children themselves; if they had not become friends their lives would probably have been very different – petty crime maybe, low-level law-breaking… who knows.

The phrase I can’t remember is similar to ‘partners in crime’; the pair have such a deep and close bond, such a committed though twisted relationship,  that they would do anything for the other. One may be dominant, but the other is not merely a dupe or puppet, but a willing and active accomplice not merely an accessory. There is a suggestion that their relationship and their acts escalate, increasing in frequency and becoming more depraved, as if they are spinning out of control, beyond any control, in a way lost in their horrific little world.

I will be so thankful if anyone can help with this… If I say it has a part to play in my next novel The Double Act, I hope I won’t spoil it for anyone who is going to read it!

Here is a link to my other e-books:

http://www.amazon.co.uk/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=lois+elsden