Shalom Joshua,

These are excellent questions, both of which touch on the conceptual foundations of the Sugya of Shnei Shevilin.

Regarding your first question, a number of Acharonim explain that the distinction between Zeh Achar Zeh and b'Vas Achas is not simply a matter of what Beis Din knows. Rather, each person who comes separately is judged independently under the rules of Safek Tum'ah bi'Reshus ha'Rabim. This idea is strongly supported by Tosfos (Shevuos 19a), who writes that if one person walked both paths and entered the Mikdash after each one (becoming Tahor in between), each entry was permitted at the time because it was judged as a separate Safek. Nevertheless, he must ultimately bring a Korban, since mi'Mah Nafshach one of the entries was certainly b'Tum'ah. This demonstrates that the later certainty does not undo the original Halachic ruling. Each case remains a genuine Safek when it is judged, even though considering both together later creates a definite obligation.

Another approach is suggested by the Magid Mishneh (Hilchos Chametz u'Matzah 2:12), who explains Shnei Shevilin in terms of Teliyah: each case is permitted because we attribute the doubtful event to the other case. This shifts the focus away from Beis Din's knowledge and toward the independent treatment of each Safek.

Your second question may depend on the broader discussion of when two separate rulings combine into a mi'Mah Nafshach. See Tosfos (Kesuvos 27a), together with the Maharshal and Maharsha there, who disagree about the circumstances under which separate cases merge into a single Halachic certainty. See also the Shitah Mekubetzes ad loc., who explains Tosfos differently and contrasts the approach with Rashi. Those discussions may provide the conceptual framework for analyzing whether two people jointly performing the Avodah of a single Korban would still be viewed as two independent rulings, or as one combined case.

Kol Tuv,

Aharon Steiner