Published On Sep 4, 2018
Boeing's Starliner crew vehicle will be the first payload to use the Dual Engine Second Stage on the Atlas V since 2004. While the 2 engine version offers better payload to orbit, it's very expensive and the actual reason for the 2 engine requirement is a bit more subtle and stems from earlier plans for post-Shuttle crew vehicles.
NASA Spaceflight Forums has a lot more on this subject, including links to many of the documents I'm citing
https://forum.nasaspaceflight.com/ind...
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