Nieman Journalism Lab in Boston.

Journalism Nieman Journalism Lab has a typically forensic analysis of how sister websites, Boston.com and BostonGlobe.com coped with in the aftermath of the bombing, retaining their own identities while providing a much need local service:

With the split of the two sites in 2011, Boston.com was supposed to become the place for local information, entertainment, and reader engagement. That proved true during the week of the bombings, as Boston.com became a kind of community message board for the city, with readers posting how they were feeling, as well as people offering up rooms or couches for displaced marathon runners. “We’ve always been an outlet where people could vent, where they could share or find other like-minded individuals to talk with. That was just magnified,” Hanafin said.

There is the potential that the twin site option could become the norm. You could foresee, for example, The Guardian keeping the content which appears in the paper also paid for online with the aggregated elements on a free basis. I'm subscribed the to RSS feeds for G2 and the weekend supplements and it's always strange when their content is published the day before or sometimes whole days. I'll read something on Saturday then be surprised when I see it The Observer on Sunday.