First, the narrator describes the clothing and possessions of Lennie and George. They are dressed the same and are, in this sense, identical. They wear denim because they are ranchers. Essentially, they are wearing the same 'uniforms.' The uniform indicates their job or their role in society. They have the same role, although events unfold showing how George is much more suited for the role.
Both were dressed in denim trousers and in denim coats with brass buttons. Both wore black, shapeless hats and both carried tight blanket rolls slung over their shoulders.
Then there is the physical description. Although identical in dress, Lennie and George are opposite in every other way.
The first man was small and quick, dark of face, with restless eyes and sharp, strong features. Every part of him was defined: small, strong hands, slender arms, a thin and bony nose. Behind him walked his opposite, a huge man, shapeless of face, with large, pale eyes, and wide, sloping shoulders; and he walked heavily, dragging his feet a little, the way a bear drags his paws.
What is unusual is that these are two men who couldn't be more different but they are together (voluntarily) and they make a living the same way.