I wonder now if part of the driver for that recent Atlas order was to move more volume uphill quickly to free up later Antares flights for the extended contract.
I believe it's rather a way to ensure smooth delivery at first. If I were to bet, I would bet on Atlas V (CRS4)->Antares 230->Atlas V->Antares 230. The first flight is sure to have some anomalies that would need to be studied and mitigated. And were anything to happen to the debut or second flight, they would have enough time to add a third Atlas V. Besides, if they can do the first Antares plus the two Atlases, that would mean 10.2 tonnes of cargo already on the ISS (plus whatever SpaceX and HTV can rush in). After that, ISS reserves should be in a much better situation.
If lead times are an issue for CRS2, they could very well add another Atlas V in the mean time.