The dressing alike may (or may not) mean that they agreed to dress like that, whereas the "wearing the same gown" has a more gossipy (that is unintended coincidence) spin to it.When two fashionable and rival ladies both appear in the same gown, it is the result of some scheme, and not happenstance.
From: An Unsuitable Duchess
The door cracks open, the sound of the seals releasing from each other, and he reveals himself to me in pieces. A foot. A hand curled around the edge of the door. A shin and a knee. The rest of him all at once. A pleasant shape, broad shoulders, slim hips, all wrapped in jeans and a white shirt. Now this is embarrassing. We’ve turned up to the ball in the same gown.http://www.newwriting.net
› Student Writing › The Bitter Red