Only one player in MLB history has hit two grand slam home runs in the same inning. That was Fernando Tatis of the St. Louis Cardinals in a game against the Los Angeles Dodgers on April 23, 1999. Atlanta pitcher Tony Cloninger hit two grand slams in one game against the San Francisco Giants on July 3, 1966. The first grand slam was in the first inning off of Bob Priddy, the second was in the 4th inning off of Ray Sadecki. Cloninger became the first player in National League history to hit two grand slams in a single game.