I don't have personal experience as a parent of an adopted child, and I am not an adopted child, but, one of my best friends is adopted, and I feel as though sharing her situation/circumstances will give you the perspective that you are looking for.
She was adopted by her parents when she was 4, and does have some memory of her life (with her mother) before she was adopted. She became a big sister to a brother when she was 6, and LOVED her brother. They were always very close. Her parents truly felt as though she was theirs, no matter how that came to pass, and felt there was no difference between their children, and so the issue of the siblings not being biologically related was not discussed. My friend and her parents didn't maliciously or even purposely keep the fact that she was adopted a secret from her brother, it was just not relevant, they felt.
Eventually, when my friend was in her 20's, her biological father, who she had NEVER known, contacted her and wanted to establish a relationship. She agreed, with her parent's blessing, and embarked on an emotionally difficult path where she eventually discovered the circumstances that led to her birth, and ultimately to her adoption. She discovered she had three other siblings, two half brothers on her dad's side and a half sister on her mom's side. It was difficult for her, understandably, but she got counseling, and did the best she could. However. Her brother, with whom she was raised, was totally blindsided by this. Not only was it very difficult for him to deal with the fact that he had been "lied to" by his family, he also felt left out once my friend began developing relationships with her newly discovered families (bio mom and dad were not married to each other-she was the product of an affair between married people.) Ultimately, she gained three new siblings, but lost her brother. Her brother is also estranged from his parents now too. Ten years later, things are finally starting to get better for my friend and her family (immediate family, we'll say). But her biggest regret, and her parents', is not discussing the impact her decision to explore her past would have on her then teenage brother. Her parents had mentally prepared HER for the difficulty, but never thought how it would affect him.
I guess the moral of this story is that whatever you choose to do-adopt first or second- the important thing is to be honest with yourself and with your children, and be prepared to navigate through some difficult waters. I hope that sharing this experience helps.