The weekend after the New York Stock Exchange delivered its bombshell about merger discussions with Germany's stock-market operator, two other exchange chiefs traded messages about the news.

"Are you thinking what I'm thinking?" said one of them, Jeffrey Sprecher of the IntercontinentalExchange. On the other end of the line was Nasdaq OMX's Robert Greifeld, a man who Sprecher had enjoyed meeting over the years, but with whom he had never done serious business.