We ended the week with a kashe of the Mishne laMelech comparing krias hamegillah and the mitzvah of haseibah. To briefly summarize: Why in a case of safeik mukfaf / safeik not mukaf do we read the megillah only on the 14th and rely on sfeika derabbanan l'kula as an exemption for the 15th, but with respect to safeik of which set of kosos require haseiba we do not say that haseiba on the first set of kosos exempts haseiba on the last set based on sfeika derabbanan l'kula.
The Emek Bracha offers the following answer: with respect to the mitzvah of krias hemegillah, since one must read on at least one of the two days, either the 14th or the 15th, the 14th takes precedence based on rov or zerizus. Once there is at least a safeik that the mitzvah of krias hamegillah has been fulfilled, no further obligation exists (since sfeika derabbanan l'kula). The same cannot be said with respect to haseiba. Haseiba is not an independent mitzvah with a safeik whether it should be fulfilled using the first 2 or last 2 kosos -- rather, haseiba is a condition in the mitzvah of the kosos. Since each kos might be a kos that requires the condition of haseiba as part of its fulfillment, the fact that the first kosos were drunk with haseiba has no effect on the obligation to meet the same condition for the last kosos.
The issue the Emek Bracha's answer raises, namely, whether haseiba in an independent mitzvah or a detail in the mitzvah of drinking 4 kosos, may be at the crux of another debate among Rishonim, as we shall bl"n see.