Ambrotype - The Watts Twins in Summer Dresses by Photo_History on Flickr.
This 1/2 plate ambrotypes shows twin girls standing on a carpeted platform. They wear identical off the shoulder dresses. In the background are sets of legs that appear to be braces to hold them still although they are not the standard upright on a heavy base.
A second ambrotype shows them in identical fur trimmed coats and hats.