We establish in the gemora that the Chachamim hold that m'safek we go lechumra for Y"T and Shabbas together both keduasha achas and 2 kedushos. Thus we would assume you can't make before Erev Y"T (before Shabbas) one eiruv that can be eaten on the first day (Y"T) because we are concerned we may need the eiruv to be chal on Shabbas as well (= chashash 2 kedushos), and we would forbid making an eiruv in a different direction for the second day kadosh becuase of a chashash of kedusha achas (as we see in the braisa brought in the gemora according to that shita of kedusha achas).
But instead the Chachamim give the "Keitzad..." which tells how to make an eiruv for the second day different from the first. What happened to the klal established in the gemora that we're choshesh in both directions?!
When the Mishna gives a case of "Keitzad..." it is not telling us how to make an Eiruv for the second day that is different from the first. It is saying how to make an Eiruv on the second day in the same spot as the Eiruv on the first day. If there is one Kedushah, the Eiruv on the first day suffices. If there are two Kedushos, the Eiruv was reestablished on the second day.