The word is a combination of the 'Yoo' in (Yoo)chun and the 'su' in Jun(su).
The name order relates information about the position each person holds in the relationship.
In Yoosu, Yoochun is the dominant partner (coming at the beginning of the name), Junsu is the less dominant partner (coming at the end of the name).
Girl#2: No WAY! Yoosu couple is so real!!! :D