Yesterday, Josh Willingham of the Washington Nationals hit two grandslams in one game, becoming the 13th player to ever do this. It was just 4 days after Mark Buehrle of the Chicago White Sox pitched a perfect game. So I wondered which event is rarer. To see all of the occurrences of these events, you can to to the following Baseball Almanac sites:
Two Grand Slams in One Game
The first player to hit 2 grand slams in one game was Tony Lazzeri, in 1936. Maybe it is not surprising no one did it before 1920, in the dead ball era, when HRs were rare. So I use 1920 as the starting point for the comparison between these two rare events. I only include regular season games, so Don Larsen's perfect game in the World Series will not count. And I also include all games where a pitcher had 9 perfect innings from the start of the game, regardless of what happened after the 9th inning. So I include Harvey Haddix's 1959 game and Pedro Martinez's 1995 game. After all, it was not their fault their teammates could not score just one run for them. They did match what these other pitchers did.
Using the data from the Baseball Almanac site, that leaves 15 perfect games since 1920. That is two more than games when a player hit 2 grandslams. There were about 149,000 major league games played from 1920-2008. So prior to this year, a perfect game happened once every 10,642 games. Two grandslams in one game happened once every 12,416 games.
Looking at the Baseball Almanac sites, you can see that in 1968, 1995, 1998 and 1999 both events occurred. So 2009 is the 5th year that both happened. But this time it was only 4 days apart. In those other years the two events were always atleast a month apart.