On May 2, 1987 — Graig Nettles and Dion James each hit grand slams to lead the Atlanta Braves to a 12 – 4 rout of the Houston Astros. It is the first time since July 3, 1966 (when pitcher Tony Cloninger did it by himself), that the Braves have hit two grand slams in one game.

Source:
Baseball Reference May 2