To illustrate, imagine you have just read Psalm 1. You ask yourself: who is this blessed man? The one-step interpreter says - this is Jesus. This description could never match anyone but Jesus. And then they will usually draw a link straight in to Psalm 2 and make the anointed man in that Psalm equal the blessed man in the other, and both of them identified as Jesus. The two-step interpreter is more likely to read Psalm 1 as a wisdom Psalm - a text which establishes the categories of blessedness and wickedness, into which all people could broadly be allocated. And then they would make the second step, pointing out that Jesus is of course the ultimate fulfilment of what it means to be the blessed man, and that this Psalm which deals in general categories only finds its grounding in human reality through Christ.
Or another example - suppose you are reading the Song of Songs (it's Solomon's, you know). The one-step interpreter says that this whole Song is about the relationship between Christ and his church, and sets out to show how the details match up with that relationship. The two-step interpreter says that the Song is an (often highly erotic) love song, telling the story of the relationship between a man and a woman. And then they would make the second step, showing that marriage itself is a picture of Christ and the church, and therefore seeing Christ in the Song.
I've been back and forth on this, but I'm now pretty firmly in the two-step camp. Here are some reasons why:
1. One-step interpretation leaves us open to the charge that we are just making stuff up. If we end up saying stuff which anyone with a basic grasp of comprehension would be able to expose as 'reading in', I think we're in trouble. So, when Moses struck the rock he was really striking Jesus was he? Then why is there no indication of that in the text? Why do we have to explain (away) so much of the Song in order to make it about Christ, or resort to arbitrary allegorising?
2. One-step interpretation undermines the uniquely revelatory character of the incarnation. When Christ came into the world, so did light - see John 1. The implication of this, and numerous other parts of the Old and New Testaments, is that the OT is full of shadows, which the one-step interpreter wants to disperse prematurely.
3. One-step interpretation seems to want to make the Scripture about Christ by denying that the Universe is about Christ. This is a bit obscure, but it's clear to me from the treatment of the Song. Is marriage - all human marriage - ultimately about Christ and the church? The Apostle says it is. Well then, what is the difficulty with saying that the Song is about human marriage? It shouldn't undermine the Christological and gospel importance of the Song in any way, unless you have a sneaking doubt that marriage really is about Christ, and feel that there is some need to short circuit this.
There's more, but I wondered if anyone had any thoughts on those?