Not two this time, but three: Gen. 12:10-20, 20:1-18, and 26:1-11. This triple play, each a story of a man protecting his wife by calling her his sister (and a comedy of errors resulting thereafter), have been gold for the JEDP theorists.
The fact that they assign Gen. 20 to E and the other two stories to J already shows the inconsistency of the critics, for they admit a doublet within the same author -- or else posit a series of Js to allow retention of the theory. .
The similarity of these stories, far from being proof of someone weaving together different sources, actually (in the context of ancient literature) proves no such thing. The use of repetition was par for the course in these contexts, and there is no reason why a single author cannot have been responsible for all three stories. One may as well posit varying authorship for the three accounts of Paul's conversion in Acts, but I have yet to hear of anyone thinking that more than one person wrote Acts...nor suggest that Luke got his three stories from three different historical-traditional sources.
Beyond that there are certain discernible patterns explained far better by the premise of a single author than by a redactor(s?) piecing together once-separate works. Garrett [Garr.RG, 133] notes within the triad "a pattern in which a narrative element is consistently present in two out of the three accounts". (He also notes the presence of another triad under the "meeting a woman at the well" motif -- Gen. 24 and 29, and Ex. 2; but no one uses these to reach the same conclusions as they do for our stories of concern.)
Many writers attempt to break up the stories in terms of motifs [cf. Gor.LWS] -- this we can certainly accept, especially under the not-too-dangerous assumption that being a wandering chieftain with a beautiful wife, and being forced by circumstances into more civilized areas, necessarily required you to do some kind of she's-my-sister deception in order to survive. Perhaps the reason the motifs evolved is because they reflected a hard reality.
Practically speaking, Hoffmeier [Hoff.WT] argues convincingly that a sort of "diplomatic marriage" custom lies behind all three stories: In order to gain the protection of the king, along with water and grazing rights, the patriarchs, having no daughters to present for such an exchange, resorted to the ruse with their wives. The threat was not therefore originally from the kings themselves, but from outside forces that the patriarchs sought protection from.
This is one point in which history sets silence to the critics: Many arguments on these stories assume that the ruler was the main source of threat.
Other than that, one author can be as easily responsible for a progression of messages (transmitted through subtle variations in the successive stories) as two or more. It's just like we've said elsewhere about the Q/Markan priority hypothesis compared with the Griesbach hypothesis: Critics will work with any literary hypothesis and derive the same conclusions they would under any other hypothesis. The literary hypothesis is just a tool that can be picked up and discarded as needed.
Not that critics don't have their own reasons for supposing multiple authors: Speiser supposed that Abraham would certainly have learned from his first mistake with Pharaoh and not done the same thing again with Abimelech; of this I can only suppose that Speiser himself never did the same (or same sort of) foolish thing twice or more. Of course under the "diplomatic marriage" paradigm, we aren't really dealing with a "mistake" in the first place but a strategem.
There is also the question of the appearance of Abimelech in both Gen. 20 and 26, which is taken to prove two different authors; but the explanation that this either involves a dynastic name, or else that this is the same Abimelech much older (Is there a clue of progress in his title changes from "king of Gerar" to "king of the Philistines"? (cf. 20:2, 26:8) Is there some sign of Abimelech as an old man in Gen. 26 in his habit of being a peeping tom?), are more than sufficient.
A minor objection asks where Jacob and Esau were in Gen. 26, as they would be at least 16 at the time and a dead giveaway to Isaac's status as a married man; this is easily resolved, for in Isaac's large party of relatives and servants there would be more than enough options for passing the twins off as someone else's (or as from another marriage).
Another objection, usually unrelated to JEDP issues, asks how a woman of 90 could be so beautiful as to be desired by the kings; Jewish tradition responded, quite sensibly, that the promise to Abraham and Sarah was fulfilled in process by reversing the aging process and making them young again -- which is indicated in our writer's hints that Sarah was beautiful at age 90, a puzzle they expect us to figure out.
A helpful reader has added this historical note: The Nuzi tablets, contemporary with Abraham, show that among the Hurrians marriage bonds were most solemn, and the wife had legally, although not necessarily through ties of blood, the simultaneous status of sister, so that the term "sister" and "wife" could be interchangeable in an official use under certain circumstances. Thus in resorting to the wife-sister relationship, both Abraham and Isaac were availing themselves of the strongest safeguards the law, as it existed then, could afford them.
So in conclusion, we can only remark that concerning these items, JEDP offers more reading of ancient literature through a modern lens.
- Bid.EA - Biddle, Mark E. "The 'Endangered Ancestress' and Blessing for the Nations." Journal of Biblical Literature 109/4, 1990, pp. 599-611.
- Garr.RG - Garrett, Duane. Rethinking Genesis. Baker, 1991.
- Gor.LWS - Gordis, Daniel H. "Lies, Wives and Sisters: The Wife-Sister Motif Revisited." Judaism Summer 1985, pp. 344ff.
- Hoff.WT - Hoffmeier, James K. "The Wives' Tales of Genesis 12, 20, & 26 and the Covenants at Beer-Sheba." Tyndale Bulletin 43.1, 1992.