Why does the Pasuk see fit to stress that, when gathering the Kahal, the Kohanim should only blow a Teki'ah? Why is it not obvious from Pasuk 3?
Rashi: Because otherwise, just as we learn two Kohanim blowing on both trumpets to prepare the Kahal to travel from gathering the Kahal, 1 so too, we would learn gathering the Kahal from preparing the Kahal to travel with regard to blowing Teki'ah, Teru'ah u'Teki'ah. 2
Rashi: From the Hekesh in Pasuk 2.
Rashi: It transpires that each group has its individual Siman: Gathering the Kahal - two Kohanim blew both trumpets, and calling the Nesi'im, one Kohen blew one, both via a Teki'ah only; whereas preparing the camps to travel, required two Kohanim blowing Teki'ah, Teru'ah u'Teki'ah on both trumpets.
R. Yehudah (Erchin 10a): One cannot learn from here, for it is a mere Siman (to gather. When it is a Mitzvah to blow, e.g. Rosh Hashanah or Yovel, they are like one; and one may not interrupt between them.)