When the Rashbam writes that they were "Chaverim," that means that they lived during the same period and thus it is possible that they argued with each other in a Beraisa.

When the Rashbam writes afterwards that Rebbi expressed that he was "childish" to argue with Rebbi Nasan, he does not mean that Rebbi Nasan was so much greater than he and thus it was not fit for Rebbi to argue with him, for we find that Rebbi argues with Rebbi Nasan in many Beraisos throughout Shas (e.g. Berachos 3b, Pesachim 93a, Kidushin 64a). Moreover, we see that Rebbi said "I was brazen with Nasan ha'Bavli ," (and not "Rebbi Nasan"), referring to him only as a colleague would. Rather, when Rebbi stated "I was brazen..." he was simply relating to Rebbi Nasan with great respect, as Rebbi's overriding characteristic was humility (as the Gemara (end of Sotah) says, "When Rebbi died, humility ceased").

Your suggestion that it was the Amora'im who were "Soseim" the Beraisa does not appear to be correct. The Tana'im, and not the Amora'im, redacted the Beraisos, and we never find an Amora composing Beraisos. When the Rashbam there writes that "Amora'im argue regarding their opinions," he means to say that Rav and Shmuel in our Sugya argued the same Machlokes as Rebbi Nasan and the Tana Kama in the first Beraisa, which is also the Machlokes of Rebbi and the Chachamim in the second Beraisa. (When the Rashbam adds that "he was Sosem this Beraisa like Rebbi," he is not referring back to the Amora'im that he mentioned, but rather to the Tana of the Beraisa, who was Sosem this Beraisa like Rebbi and was Sosem the other Beraisa like Rebbi Nasan.)

M. Kornfeld