The relationship between Biff and Willy can be described as turbulent, dysfunctional and, actually, dissociative.
It is turbulent because the history of angst, secrecy, and disappointments between Willy and Biff prompts instant arguing and fighting between the two men, at any given time.
The angst on Biff's part comes from the disappointing discovery of his father's affair with "The Woman." This discovery occurs during a time in Biff's life when his high school successes have just come crashing down, and he needs the support of the very man who builds him up and puts him on a pedestal.
Knowing that this very man, his father, is capable of lying and deceiving puts Biff in a diatribe: IS he the "wonder child" that his father, the liar, has made him out to believe he is, or is Biff Loman yet another one of Willy's "lies"?
On the other hand, the fact that Willy knows how Biff feels about him after the discovery is a huge bad blow to his ego: he no longer has his "wonder child" to boast about, and he can no longer continue his project of making Biff everything that Willy wished he could have been. Hence, the men's communication falls apart and seems to only lend itself for conflict.
The relationship between Biff and Willy is dysfunctional because it lacks the defined boundaries and limitations of affection and mutual respect that should exist between a parent and a child. In a healthy parent/son relationship the father nurtures the son and lets him develop according to the child's own possibilities. Willy does this, but not altruistically: he builds Biff as a more handsome, more talented, and more successful version of Willy in order to vicariously re-live his lost years.
As a result of the lack of proper boundaries between father and son, Biff sees his father as his rival and enemy during trying times. Rather than trying to seek a reconciliation that would enable them to move forward, they basically go head-on against each other like if they were not even related. This is the epitome of a dysfunctional parent/child dynamic.
Finally, the relationship is dissociative because both men have lived, either in denial or in complete dissociation, of the reality of their lives. Their lack of focus on themselves as individuals, makes it impossible for them to view themselves as part of a healthy relationship. Willy sees himself as a hot shot salesman, sees Biff as a hot shot football player, and sees his American Dream fulfilled. As a result, Biff starts to be sucked into Willy's dream and ends up believing the same things...until reality hits Biff and he realizes that his life had been a lie, all along. Hence, Biff moves out West trying to find himself. However, as he himself says, "something" always brings him back.
Therefore, dissociation, turbulence and dysfunctional roles constitute the backbone of the Loman's relationship. It is a relationship that can only be healed by the a reality check, and by the mutual choice of accepting it the way that it is.