Jenson notoriously denies the logos asarkos , insisting that the pre-existent one is none other than Jesus Christ. It’s a confusing notion, but in his discussion in Systematic Theology: Volume 1: The Triune God (140-1), it’s a fairly straightforward implication drawn from a Barthian notion of election.
For Barth, “God is so unmitigatedly personal that his free decision is not limited even by his ‘divine nature’: what he is he himself chooses.” This can only mean “God is the act of his decision.” But His decision in eternity is to “unite himself, in the person of Christ, with humankind; he chose to be God only as one person with the man Jesus.” Since the decision determines His being, “he is the man Jesus.”
Jenson disputes “Barth’s total construction,” but maintains Barth’s basic point: “‘Before’ the existence of the Incarnate there is the eternal triune life, in the actuality of which it is decided that there be a created history and a life of the Son in that history.” The second “identity” within that triune life must be the “same Incarnate Son,” following Barth’s logic.
Jenson acknowledges that there is a sense in which “the one Jesus Christ as God precedes himself as man, in the very triune life which he lives eternally as the God-Man.” Appealing to the relation of Old and New Testaments, he suggests that “the Son appears as a narrative pattern of Israel’s created story before he can appear as an individual Israelite within that story.” The actual prior identity of the Son is “the narrative pattern of being going to be born to Mary,” the narrative pattern Jenson expounds at the beginning of his book in tracing the patterns of trinity and incarnation in the Old Testament.
This is not intended at all to defend Jenson’s conclusion, only to show the logic that leads to it.