joined at the hip
(idiomatic, informal) Closely connected, as in an intimate friendship.
joined-at-the-hip
Aaron and I will be joined at the hip until the day we die. We have loved and hated each other since the day he was born. He's very much a part of my heart. He's going to broadcasting college now, and he'll do fine. But he came into a world that did not welcome him. Lynn Johnston
I spend so much time with my parents. My mom and I were joined at the hip for five years. There was not one moment when I wasn't with her. Kay Panabaker