Dual-core family refers to a family where both husband and wife are the only child or the single “core” of their own families before they got married. The new family, of course, has two “cores” as neither of the couple knows how to behave not as a “core” in a family.

“双核家庭”指夫妻双方均为独生子女的家庭，双方在婚前都是各自家庭中的“核心”，因此称为“双核家庭”。在两人建立的新家庭中，双方都不知道该如何以“非核心”的方式行为处事，于是，家里就出现了两个“核心”。

For example:

The number of dual-core families has been growing in recent years since more and more post-80s are getting married.

随着越来越多的80后走入婚姻殿堂，双核家庭的数量在近几年不断攀升。