"Whether or not one can determine the motivation for joining Gen. 1 with Gen. 2, the present juxtaposition within a larger literary context affects the semantic level on which ch. 2 is read." (JSOT 16, 1980: 54).Yet, at the same time, he does believe that some sort of intentionality is vital for the canonical approach, such that our interpretations should be 'coerced' by it (e.g. in this article) I've just re-read Seitz's intro to Word Without End, and I feel that I'm on the road to understanding what the nature of this 'intentionality' is.
The 'canonical intentionality' of the final form includes the discreet intentionalities of the tradents, yet is at the same time of a different order (or at a different level). It goes beyond them somehow. This, at first, seems an odd idea (Barr called the concept 'magical'). However, a truly theocentric reading (which is how Childs sells his idea) is interested not in the text itself, but in the reality (res) to which the text points. This reality is (arguably) theological and is outside the text (hence allegory over midrash for Christians). In other words, the truth to which the text witnesses is greater, richer and more complex then the individual authors could have perceived. It encompasses the text, author and reading community.
The various tradents of the traditions did their work in reponse to this one reality, such that they participated in it while never comprehending it fully. To the degree that the tradents submitted to this reality, their combined messages were consistent with it while never fully comprehending it. It would then seem logical that even traditions which were accidentally brought together (a reality Childs affirms) would speak of this truth more adequately when heard in concert. To get back to the author's conscious intention is to make an anthropocentric move, one which from the outset assumes revelation is not a case of an external reality evoking a response from a people united by covenant and faith. This qualifies Childs' use of 'reader response' theory. The reader is responding to something that was 'intended', though ultimately it God who is intending and not the individual authors.
This assumes that there is a single reality behind the diversity, thus ensuring that canonical interpretation can only really be done by those who believe it in the first place. A secular approach will struggle to comprehend a move that goes beyond concrete, historical intentionality (including the postmodern variety, which focuses on our concrete, historical intentionality).
Does that make sense? Feel free to tell me I'm missing the point ...
UPDATE: Murray Rae wrote an excellent essay on this topic in the first edition of the Journal of Theological Interpretation. It's entitled "Texts in Context: Scripture and the Divine Economy" and can be read here (after Joel Green's short intro).