Why is this called Sheni?
Rashi: It is second of the seven in the lottery based on the portions that the Sheluchim made. Yehudah and Yosef already took beforehand.
Why did Shimon receive amidst Yehudah?
Bi'ur ha'Gra: Yakov cursed Shimon and Levi, respectively "Achalkem b'Yakov, va'Afitzem b'Yisrael" (Bereishis 49:7). Shimon receives his Chelek (portion) amidst another Shevet 1 , and Levi is Nafutz (spread) among all his brothers
Targum Yonasan (Bereishis 49:7) says that Shimon will inherit partially amidst Yehudah, and partially amidst the other tribes. This can help explain why a verse says that Yaravam received 10 Shevatim (refer to Melachim I, 11:31:1:1-3, 11:31:1:3*). However, the verses here and all the Meforshim here, including Targum Yonasan, imply that Shimon inherited only amidst Yehudah! This requires investigation. (PF)
Like Rashi explains verse 9. (PF)