Part of what led to the company's doing so many collaborations was Jim thinking about how we could grow our younger authors (people like Elizabeth Moon) and get them up to the level of shipping that they deserved, faster than just by publishing a book a year. He remembered what Larry Niven & Jerry Pournelle had done in their collaborations, and he thought he could reproduce their method and have a similar system work for our younger writers, paired with established older writers who didn't necessarily have time to write all the plots they wanted to write. We started out with David Drake, who plots like no one else -- he doesn't think it's hard! For him, it's like putting on his shoes in the morning. I'm not a writer myself, but I've met plenty of writers who say, 'I'd cut off my left foot to plot like he does.'
Around the same time, I was reading Aristotle's Poetics and realized there's a philosophical underpinning for this method of creating fiction. I recommend The Poetics for anyone who is doubtful about this way of doing things. It's similar to the way the Great Masters worked, the painters in the Renaissance. They would create the outline of the painting, their apprentices would fill in the details, and then the masters would come back and make the finishing touches that made it a brilliant painting. This is the same way that our 'arranged marriage' collaborations work (though not all the marriages are arranged; some of the writers come to us as 'couples' already). David Weber and Steve White's first novels were collaborations, for instance.
The specific aspect of the method that finds a philosophical underpinning in Aristotle is not the collaboration but the structure of it. Aristotle holds that the events of a tragedy consist plot and episodes. Tragedy, according to Aristotle, is an imitation, a mimesis, of action and life, and therefore it is primarily governed by its plot, which is the ordering of its actions. Plot is the soul of a tragedy, and exercises its proper functions when it is unified, complete, and plausible. This plot, as Aristotle understands it, is not the whole story of the work; it is the structural framework of the whole, consisting of the essential turns of fortune (the Greek word for such a turn of fortune is a catastrophe, an overturning). One also requires episodes. An episode is literally a parenthetic narrative, whatever happens to occur between the choral commentaries that structure a Greek tragedy; it is what goes with the plot without being the plot. Aristotle illustrates the point humorously in a very famous comment about the Odyssey:
A certain man is absent from home for many years; he is jealously watched by Poseidon, and left desolate. Meanwhile his home is in a wretched plight--suitors are wasting his substance and plotting against his son. At length, tempest-tost, he himself arrives; he makes certain persons acquainted with him; he attacks the suitors with his own hand, and is himself preserved while he destroys them. This is the essence of the plot; the rest is episode.
The compression here is deliberate; on Aristotle's account the Odyssey has very little plot. It's an epic, and epics because of their length are more episode-heavy than tragedies; but the Odyssey is also more episodic than most epics. Much of what you find in the Odyssey is not actually required for the major turns of fortune in the story that is the Odyssey itself, and could be broken up into little independent stories and scenes that need have nothing to do with coming home to Ithaca. This is fine for an epic; a tragedy structured like this would have what Aristotle calls an episodic plot, which he regards as defective. Amateurs write such plots because they are weak at plotting, and those with experience only write them to please the actors: acting itself is almost purely episodic in nature, because it is the episodes, not the plot, that let the actors show their skill. (This is why highly visual media of our day -- television, movies -- are so big on character arcs. Character arcs are pseudo-plots consisting of contrasting episodic representations of character in incidental relation to the real plot; acting is very well suited to this, as Aristotle recognized, but our visual media are much more intensely actor-focused than Greek drama was.)
Because of this, Aristotle's recommendation for poets is that they write their tragedies beginning with the plot, then assign names to the characters, then fill it in with episodes that are interesting and relevant to the plot. Aristotle doesn't envisage this as collaborative, but the Baen method is, as Weisskopf suggests, Aristotle's method, just with a division of labor.