Because they're both open to traveling the campaign trail together.
Sen. Barack Obama said Saturday that if he were to become the Democratic nominee, holding joint town hall-style campaign events with Republican Sen. John McCain would be a "great idea."
"Obviously, we would have to think through the logistics on that," Obama continued. "But if I have the opportunity to debate substantive issues before the voters with John McCain, that's something that I am going to welcome."
Recently, advisors to the all-but-certain GOP nominee have said the Arizona senator is open to the idea, and his campaign has touted the fact that he and Democrat Bill Bradley held joint campaign events when the two ran for the presidential nomination in 1999.
Obama is better when he commands a room by himself — he is, as everyone knows by now, an impressive speaker. McCain is not, and these joint town halls would definitely play to his strengths. One gets the feeling that David Alexrod might pull Obama aside sometime soon and put the kibosh on this idea.
Update: Noam Scheiber agrees, and adds that joint town halls would give the cash-strapped McCain lots of free media.