In mitral stenosis, double atrial shadow is due to

The double-density sign, also known as the double right hea border, is seen on frontal chest radiographs in the presence of left atrial enlargement, and occurs when the right side of the left atrium pushes behind the right cardiac shadow, indenting the adjacent lung and forming its own distinct silhouette If large enough, the left atrium can actually reach beyond the border of the right atrium; this is known as atrial escape. The right superior pulmonary vein can lead to a similar appearance . Ref Davidson 23rd edition pg 480