Families with more than 1 adopted child

Hi! I am new to the forum. We have a son whom we adopted domestically at birth. We are currently a "family-in-waiting" for our second child. Have any of you had any experience, or do you know of any research I can review, regarding families where there was an open adoption with one child and a closed or semi-open with another child? Though we were open to an open relationship with our first son's birthfamily, our "relationship" with them has been purely through letters and pictures on our part only, which we send to the agency each year. With the new child we have gone into the adoption seeking more of a closed adoption to mirror the scenario with our first son's birthfamily. We are trying to be mindful of any feelings that might be generated in him given that his birthparents don't want any contact. We don't want him to feel hurt that the new child has a relationship with their birthfamily while he doesn't with his. But perhaps we are foreshadowing unnecessarily?? This is unchartered territory for us and while we want to be mindful of first son's feelings, we also don't want to unnecessarily limit our exposure to potential birthmothers based on a perception on our part that our first son might be hurt. Any insight or literature you can direct me to would be greatly appreciated. Thank you!