I'd say that for it to be a United States instead of just a 'greater Virginia' or whatever, it needs to have at least three out of Virginia, Pennsylvania, New York and Massachusetts as these will result in the guarentee of a confederal/federal structure due to the balanced power. Having just two makes it more of a dual republic and likely to split apart.
So the bare minimum would probably be Virginia, Delaware, Maryland, Pennsylvania, New Jersey and New York for a 6 state Union. Though if Massachusetts has been retained it's more likely for New York to have been split leading to an unstable Pennsylvania-Virginia duopoly, so it might be more plausible to have a Massachusetts-Pennyslvania situation, possibly including Maryland, but probably an 8 state situation of PA, DE, NJ, NY, MA, RI, CN and NH, with or without VT. Delaware included because I honestly can't see an agreement that would lead to Philadelphia being so vulnerable to attack as having the Delaware River contested like that. Capitals would probably be remaining at Pennsylvania for the first case and New York for the second due to centrality.