Two Men

D.D. Harriman is the title character of Robert Heinlein's story, "The Man Who Sold The Moon," which is the title story of his collection, The Man Who Sold The Moon, which comprises six of the twenty four installments of Heinlein's Future History and ends with "Requiem," about Harriman's death. (The second installment, "Let There Be Light," has a Biblical title.)

Nicholas van Rijn is the title character of Poul Anderson's collection, Trader To The Stars, of his novel, The Man Who Counts, and of the posthumous omnibus collection, The Van Rijn Method, which collects the first eleven the forty three installments of Anderson's Technic History, including The Man Who Counts and the first of the three stories originally collected as Trader To The Stars.

These two men play comparable roles in their respective future histories. It is possible for them to meet because both van Rijn and Harriman's contemporary, Rhysling, visit the inter-universal inn, the Old Phoenix. (Heinlein also has a version of the multiverse but that I emphatically discount.)

Sandra Tamarin rightly tells Eric Wace that he and she could not have escaped from Diomedes without van Rijn but I think that she exaggerates when she also says that:

"'I think, without us, he would have found some other way to come home...'" (The Man Who Counts, XXII, p. 290)

Of course van Rijn would have tried and might have succeeded but this is not guaranteed. He could have been killed several times when he was with Sandra and Wace. The whole point of a competitive economy like that of the Polesotechnic League is that some succeed whereas others fail. Failures do not usually make for enjoyable stories but, e.g., the trade pioneer crew might arrive at a planet where a previous team came to grief and it is necessary to deduce what went wrong. The failures are part of the story.