Bob Weir and Keller Williams bonded on the traveling So Many Roads festival in 2001 and have remained close ever since. Now Williams will once again spend a number of weeks on the road with RatDog and its temporary guitarist Steve Kimock, who is filling in for an ailing Mark Karan. On Saturday the friends kicked off what is sure to be a collaboration heavy summer at Atlantas Chastain Park Amphitheatre, where Weir emerged during Williams set for versions of ‘Dark Hollow’ and, fittingly, ‘On the Road Again.’ RatDog and Williams will appear, and likely collaborate, tonight at New Yorks Central Park SummerStage.