In Death of a Salesman, chance takes the form of the opportunity to reach the American Dream, and the illusion that it is near enough to get it. This is evident in the way that Willy lived his life and career: Taking chances. He took chances at taking a mistress, he took chances as his job requires as a salesman, and all these shots in the dark that he kept taking were in fact to fulfill his own ideal of what "having it all" meant.
However, coincidence met with chance in those very moments: By "coincidence" Biff decides to see his Dad and ends up finding out about his mistress. In the same way, by coincidence, the market apparently had change things so radically that Willy lost his talent and skill as a salesman. Coincidentially as well, Biff and Willy simply kept passing each other by as reflections of one another and still continued being unable to call a truce.