The Inklings and the Synoptic problem

The two source hypothesis goes basically like this: Matthew and Luke made use of Mark, which they supplemented with additional sources. 

There's certainly some truth to that, but it can be misleading. It's frequently presented as a vertical model of literary or conceptual information-sharing, based on order of publication. If Mark was published first, while Matthew and Luke show familiarity with Mark, then they were literarily or conceptually dependent on Mark.

But the question of literary or conceptual dependence can be more intricate and intractable. Consider the Inklings. Tolkien and Lewis both took a keen interest in Nordic/Teutonic mythology. Likewise, Lewis, Tolkien, Williams, and Barfield all took a keen interest in the Arthurian mythos. And there were primary sources from which they drew. 

Conversely, there was horizontal information-sharing as they bounced ideas off each other, and shared drafts with each other. They influenced one another.

But that raises a tricky question: when you find Arthurian or Nordic/Teutonic motifs in their writings, what's the source? It is primary source material? Or did one Inkling get this from another Inkling? If two Inklings have the same motif, what's the direction of borrowing?

The order of publication is inconclusive. One Inkling might be the first to publish a story using that motif, followed by another Inkling publishing a story with a parallel motif. But even if we find "synoptic parallels" in writings of the Inklings, publication order faults to demonstrate that the author of the later writing borrowed from the author of the earlier writing. On the one hand, there's the possibility that both used a common source. On the other hand, there's the possibility that the author who published first borrowed the idea from an unpublished source. That is to say, that might reflect horizontal information-sharing rather than vertical information-sharing if he originally got the idea from a fellow Inkling during informal conversation. In some cases there may be letters or diaries that enable us to retrace the genesis of the idea, but in many cases, it isn't possible to reconstruct the creative process. 

In application to the Synoptic problem, in some cases it could be due to independent access to a common source. In other cases, an earlier publication might be indebted to the author of a later publication. 

Although Matthew's writing can't be a source for Mark, Matthew the writer might possibly be a source. A writer preexists his writings. The writer of a later writing can be a source of information for an earlier writing by a different author. 

Again, consider the Inklings. Even where there's evidence of borrowing, publication dates are not a reliable indicator of the direction in which that took place.