Answering this question requires considering the drastic difference between these two characters and the fact that they only meet in Willy's hallucinatory flashbacks, not in real life. Let's start there: Ben and Biff never had a "real" relationship. However, the play does definitely place these two characters into a symbolic dialogue with one another. I'll clarify some key points about each character first; then we'll look at how the two relate.
Biff is Willy's oldest son. Biff's defining feature is his struggle between who he is and who he's been raised thinking/believing he should become. His father's fixation on a successful run in business and the belief that one must be "well liked" have created in Biff a strong sense of being lost. However, we also hear a lot from Biff about how he'd rather be outdoors, with his shirt off, doing manual labor. He has no knack for business, though he feels guilty for never being able to succeed in that lifestyle, especially since he utterly idolized Willy up until his last year of high school.
Rather than a flesh and blood character, Ben is really more of a symbol, signifying Willy's vision of ultimate success. Ben's famous recurring line is "William, when I walked into the jungle, I was seventeen. When I walked out I was twenty-one. And, by God, I was rich!" (eNotes Death of a Salesman character analysis). This is what Willy wants for his boys, especially Biff--to be able to walk into the "jungle" (let's infer: New York City is often referred to as "the urban jungle") and come out rich in a short, effortless amount of time, which is true success in Willy's eyes.
We can think of Ben as representing what Willy hopes for and pressures Biff to become. Biff, meanwhile, tries to emulate this idea, but finds that it ultimately kills his soul. We see this idea play out in Act I, when (within Willy's hallucination) Ben cajoles high school age Biff into boxing with him. Suddenly, Ben takes a cheap shot and "Trips Biff, and stands over him, the point of his umbrella poised over Biff's eye" (Miller 49). Of course, Biff is surprised by this. Ben explains his sudden violence with "Never fight fair with a stranger, boy. You'll never get out of the jungle that way" (49). If we look at this symbolically, we see that Biff is told he must trade his integrity for success in business. Also, we get the feeling that interacting with Ben (who stands for Willy's vision of what Biff should become) could be potentially harmful, violent, and maybe even deadly for Biff. Biff himself knows he can't make it out of the symbolic jungle, as he finally admits to Willy in Act II:
"What am I doing in an office, making a contemptuous, begging fool out of myself, when all I want is out there, waiting for me that minute I say I know who I am?" (132).
Biff struggles and breaks free of the ghost of Ben's example in this final confrontation with Willy. In contrast, it's the ghost of Ben's example that convinces Willy to commit suicide to provide some capital for the business ventures that Biff will never actually pursue. While Biff can ultimately say that he doesn't want to become Ben, Willy cannot accept that, and clings to the idea until his death. One could argue that Ben's final reference to Biff, calling him "outstanding, with twenty thousand behind him," is really the voice of Willy.
Miller, Arthur. Death of a Salesman. New York: Viking Press, 1968.