In the play "Death of a Salesman" how are Biff and Willy idealists?

Biff and Willy both look to each other for inspiration. This, I believe, remains true for Biff even after he becomes disillusioned with his father after discovering Willy's infidelity in Boston. Later in his life, he still works to fulfill his father's grand expectations for him, such as talking about going into a sporting goods business with his brother, Happy. However, by the end of Act Two (which is when Willy dies), he gives up...

Biff and Willy both look to each other for inspiration. This, I believe, remains true for Biff even after he becomes disillusioned with his father after discovering Willy's infidelity in Boston. Later in his life, he still works to fulfill his father's grand expectations for him, such as talking about going into a sporting goods business with his brother, Happy. However, by the end of Act Two (which is when Willy dies), he gives up -- accepting that the reality of who he is and his father's hopes for him are irreconcilable. 

Because we learn about Biff as a young man through his father's recollections, which are unreliable, we cannot be completely sure about who Biff really was in his high school years. We do not know if he was truly as beloved as Willy remembers, or even if he idolized his father to the extent to which Willy remembers. If we are to believe Willy's recollections, however, it seems that, for Biff, Willy was a masculine ideal: a capable businessman, a great talker, and a well-liked man.

After he discovers his father's infidelity, this ideal dissolves. Biff's response to this disappointment is to leave New York and to go out West to start a farm. In this play, Miller uses applies to the West all of the tropes and associations that the region bears in popular culture: it is the place where people go to start over, the place where men go to create their own fortunes. Biff fails, though, even in this idealized space. 

On the surface, Biff's ideal for success is really influenced by his father's expectations. After Willy's death, he is free to abandon the ideal. He knows that he will never run a sporting goods business, or be a "number-one man." Willy's ideal is for his son to be the success that he never was. 

On a deeper level, one can say that Willy and Biff are examples of how the American Dream is unrealistic. The dream is that every man (and, in the context of this play, particularly, the fantasy is very masculine) can succeed using his individual talents; and that every man has the potential to create his own fortune. This does not turn out to be true for either Willy or Biff. Neither is particularly talented, and both make bad choices.