If she's building the relationship with DH2B and wants to call him Daddy too, why not let her do that? As long as you keep the distinction between the two of them, why can't she call both of them the same thing in separate settings? It sounds like she's putting it together logically in her young mind in a very mature way. She sounds very intelligent for a six year old and that's a good thing. Better to have extra love than not enough anyway...