For two people who are inseparable

Ser uña y carne

sehr OO-nyah ee KAR-neh — To be inseparable (literally: "to be nail and flesh")

"Lucía y Pablo son uña y carne desde el colegio."
Lucía and Pablo have been inseparable since school.

Like a fingernail and the skin under it — impossible to pull apart. Usually said of friends, but you'll hear it about siblings and couples too.

"I love this one — such a vivid image, and English doesn't have anything quite like it."— Graciela