Shalom, a couple who have problems having children due to medical complications, this couple adopting a son and a daughter will be fulfilling the mitzvah of having 2 children a boy and a girl?
Although it is not fulfilling the mitzva in the general way, however in a certain ways, and according to some poskim it is considered by shomayim to have birthed them. The gemora says that a person that raises an orphan in his home is considered to have bore him. See the following article, https://dinonline.org/2020/01/03/adoption-in-halacha/ which expands on this topic.
Megillah 13a, Sanhedrin 19b, Chochmas Shlomo E:H 1-1, Ramban Bereishis 46-17, Chazon Yechezkel -Introduction to Mesecet Yivamos, Shut Yaavetz 1-165