How will the men pass in front of the Aron?
Rashi: They will enter the Yarden with it, and cross until in front of the Kohanim.
Radak, Malbim: Initially they will stay with Yehoshua, while everyone else crosses. Yehoshua stayed, to ensure that the water remain standing until everyone crosses. After, he told the men to pass in front of the Aron, pick up rocks and take them with them.
Hashem did not command "Ish Even Achas Al Shichmo." Why did Yehoshua say so?
Malbim: One could have understood Hashem's command that they carry the rocks together, on poles. Then, they can carry more weight (Sotah 34a). Yehoshua chose that each carry a rock by himself, for each Shevet is a matter unto itself.