There is one man with light blue jeans on, and one with dark blue jeans on that are walking down the street.
Two men are wearing jeans.
'Two men' is a collective rephrasing of "There is one man with...and one with...", also wearing jeans is an alternative way of saying with 'jeans on'.
If one man is wearing light jeans and one man is wearing dark jeans, that means there are two men wearing jeans.
one man and one man makes two men.  jeans on is another way to say wearing jeans.