There have been, very roughly, about 400,000 Major League games played in the history of Major League Baseball dating back to circa 1870. 23 of them have ended as perfect games. That comes out to roughly 0.006 % of all games. Translation: I would need to see about 1,700 games to have an odds on statistical chance of witnessing one such a game (Major League teams today play 162 games per year). This 1700 is also misleading, because this means that I would have to predict the stadium it would occur, a 1-in-16 chance itself. I probably have this math off a bit. Jonah may correct me.
At the rate of seeing maybe five games a year, that means I should see one, in person, roughly once every 350 years. This is likely more accurate an idea of the rarity of this event.
Perfect games are rare events. The are special events in baseball. Between 1908 and 1922, there were none. Between 1922 and 1964, there were none in the regular season (Don Larsen threw a gem in the World Series; the only one ever pitched in the post season). 11 of the 23 have come since 1990. Are they getting more common, or are the past roughly 20 years a statistical anomaly? It may be too early to tell.
Nonetheless, given that there are now 32 Major League teams (a number quite variable over time), the chances of one stadium hosting two perfect games in the same season is really a statistical blip. But one of the beauties of baseball is that despite 140+ years of just about everything happening*, there is always room for something that hasn’t happened before.
In a season that has seen a surreal THREE perfect games (there were two in 2010, and the last time there were two in the same season was 1880! There is no other instance of five being thrown in two years), two of them took place at Safeco Field in Seattle. King Feliz Hernandez spun a 1-0 squeaker of a perfecto over the Tamp Bay Rays today, just a little less than four months after Phillip Humber of your Chicago White Sox pitched a perfect game there over the Mariners. That means there may be a few Seattle fans who are living witnesses to two perfect games (a few Yankee fans probably say David Wells and David Cone pitch their perfect games a year apart at Yankee Stadium, but …. Yankees, who cares. Not many people can claim that.
*Note: while most things indeed have happened over the course of baseball history, a Cubs World Series win in the last 100 years remains on the list of the impossible. The United Nations annually assures the global community of peace loving nations that it is taking steps to prevent this from happening, in order to preserve the greater world brotherhood and dialogue between nations.