A man in a red shirt and a man in a brown shirt smiling at the camera and holding up a sign.
Two men display a sign to the camera.
A man and a man are two men.  Display is a rephrasing of holding.
A man and another man are two men in total, and they are holding up a sign while smiling at the camera, so they are displaying a sign to the camera.
Holiding up a sign can be described as displaying a sign.