The same two characters, said with a light second syllable. The elder half stops counting here: what is left is the younger brother alone, or whoever is being spoken to as one.
I have only just arrived, so please all bear with me.
Tips
usage
Spoken Chinese uses 兄弟 three ways: for a man's own younger brother, as a warm way to address a younger man, and as a modest way for a man to refer to himself in front of a group. The last one attaches straight to a pronoun, 兄弟我, meaning yours truly.
mistakes
The second syllable decides which word this is. Read neutral, 兄弟 is this colloquial word and points at one person. Read with a full fourth tone, 兄弟 means brothers as a set, and only that reading takes a number in front of it.
