On May 18, 1950 — At the Polo Grounds, Rube Walker hits a grand slam in the 6th inning for the Chicago Cubs. In the bottom of the inning, Monte Irvin also hits a grand slam for the New York Giants. It is the first time in major league history that each team hit a grand slam in the same inning. The game is called on account of rain after six innings, and the Giants win, 10 – 4.
Source:
Baseball Reference May 18