The Star is much lighter now - gone is the heavy Heritage era baggage car, which has been replaced with a lightweight Viewliner Baggage. And there is no heavy Heritage Diner, it's been taken off the train. And lately just one Viewliner sleeper verses two, or even three, in the past. Just seven passenger cars and no older Heritage cars to weigh down the consist.
So one engine should be fine, especially when you consider that the long running level route to Florida has very minimal changes in elevation. And I would be willing to bet there may be some savings to Amtrak by having one engine verses two.