Why exactly, did Rachel call her son 'Yosef'?
Rashbam: For two reasons - because Hashem had gathered in (hidden) her shame, but then, he ought to have been called 'Asaf' - so she replaced the 'Alef' with a 'Yud Vav,' to hint at the second child that she hoped to bear 1 (or prophesied that she would).
What made Rachel think that she would bear another son?
Seforno: She assumed that she was granted her first son on account of her first Hishtadlus (refer to 30:22:1:2), and that she would be granted a second son on account of her second Hishtadlus.
Tosfos ha'Shalem (4): The son that was changed inside [Leah] to a girl, was in order that I can have two sons. 3
Tosfos ha'Shalem (3, citing R. Efrayim): She expected to be no less than her Shifchah.
Oznayim la'Torah: When she gave Bilhah to Yaakov and declared "ve'Seled al Birkai," (30:3) promising to bring -up her son as her own, she knew that Hashem would repay her 'Midah k'Nneged Midah,' and grant her a son. And she felt certain that, now that Bilhah had given birth to a second son, whom she would also raise on her knees, Hashem would repay her with a second son.
Which explains why she prayed for only one more son.
Perhaps she knew this through Ru'ach ha'Kodesh. Or, the Imahos (Yaakov's wives) had gathered to pray for this (Bereishis Rabah Sof 72). Or, Rachel could have known that initially her own fetus was a girl (refer to 46:15:1:1; see also Tosfos ha'Shalem to 30:22 (2)), and understood that Hashem switched the fetuses to enable her to have two sons (PF).
What are the implications of "Ben Acher"?
Ha'amek Davar, Oznayim la'Torah: She saw with Ru'ach ha'Kodesh that the second son will be unlike Shevet Yosef. Binyamin was drawn after Yehudah in every way (whereas Yaravam, from Yosef, led the other tribes to separate from Yehudah - PF)!