How do two people write a mystery? (Books - The Locked Room by Maj Sjowall and Per Wahloo)

Classes have started and let me just say, not with a whimper. So my pleasure reading is likely to slow down a bit. Hopefully I'll be able to average 5 books a month. I don't know why that's so important, I thinkn it's a quality of life meter for me, or something. Anyhoo, Maj Sjowall and Per Wahloo are my current fiction choice. The Swedish husband-and-wife team created a much admired series of mysteries in the 1960s and 70s with detective Martin Beck. The Locked Room, which I am about half way through, is a real puzzle. A man is discovered shot to death in a room that is completely locked and without a weapon. This is a dark little book, written in straight forward, almost grim prose:

Martin Beck felt his first working day was at an end. Tomorrow he'd go and have a look at this locked room himself. What was he to do tonight? Eat something, anything, and then sit leafing through books he knew he ought to read. Lie alone in his bed and wait for sleep. Feel shut in.

In his own locked room.

The mystery is set in an era that follows a nationalization of the Swedish police force in the mid-1960s. They are critical of Sweden's social system, of the new force's leaving tens-of-thousands of crimes unsolved and particularly of the arming of Sweden's police.

All of a sudden, situations that formerly could have been cleared up by a single man equipped with a lead pencil and a pinch of common sense required a busload of patrolmen equipped with automatics and bullet-proof vests.

The long-term result, however, was something no one had quite foressen. Violence breeds not only antipathy and hatred but also insecurity and fear...

Let's just say that Sjowall and Wahloo give you their moralizing straight-up, no chaser.

What I want to know is how two people write a mystery. Did they alternate chapters or what? It has that feeling - pockets of reportorial context, sections of swift-moving plot, and some tremendously awkward dialogue in this translation by Paul Britten Austin. Does an inept cop with the name of Bulldozer really say:

So do you maintain you haven't met Malmstrom and mohren or even heard from them in the last six months?

But I am going to try to stick it out. The puzzle has me intrigued and I want to see whether the authors' moral stance on the Swedish police is just a pet peeve that made its way into the novel because they couldn't help themselves or because it contributes to the story.