1)
What is the meaning of "v'Alav"?
1.
Rashi (citing Targum Onkelos): It means 'next to'. 1
1
Sifsei Chachamim: Rashi did not need to explain so in Pesukim 5, 12, for there it says "veha'Chonim Alav." Refer to 2:20:151:1.
2)
Why it does not say "veha'Chonim Alav", like it says in Pesukim 5, 12 and 27?
1.
Moshav Zekenim citing R. Yehudah ha'Chasid: It is because Efrayim and Menasheh resemble one Shevet.
2.
Moshav Zekenim: Every Shevet had a river coming out of Miryam's Be'er, [that supplied it with water]. There was only one river for Efrayim and Menasheh.