Why doesn't rama shoot his arrow to sugriva when sugriva and bali fight for the first time?

He can't tell which one is Bali, because the brothers look alike.

Sugriva was the younger brother of Bali and they were actually identical. Rama did not shoot his arrow because of this, he was afraid he would mistake Bali for Sugriva. Rama preferred Sugriva over Bali and wanted to help him raise to power, but he needed Bali dead so he arranged another confrontation after this failed first one. He had an idea, he will put a garland on Sugriva to be able to identify him and shoot Bali with his arrow while they were fighting, he succeeded.