What he's saying here is that this merging of minds is how transmission from mind to mind works, and it is twining vines. When we come together as a sangha, we merge and influence each other. What Dogen is really talking about here is the same principle that occurs when teenagers get together. They all start talking alike. Teenagers exchange personas in little bits and pieces and are becoming new people by their contact with others.
Teenagers are wonderful to watch.
Something in me arises to meet what is present. It doesn't matter whether it's pleasant or unpleasant, whether I'm being challenged or receiving adulation. None of that matters. Still, something arises in me, so that I am actualized by the person with whom I am coming into contact. It's through this exchange of psyche, that takes place quite naturally, that something of the Dharma can be transmitted.