faire d’une pierre deux coups

To achieve two things with a single action

Used whenever one action or trip sorts out two problems at once, much like the English "two birds, one stone".

En allant chercher les enfants à l’école, j’ai fait les courses en route, ça faisait d’une pierre deux coups.

While picking up the kids from school, I did the shopping on the way, sorting two jobs at once.

Goes back at least to Montaigne, who wrote a version of it in the 1570s. Nobody's sure exactly why a stone and two hits became the image, though a popular guess involves a slingshot lucky enough to bring down two birds at once.