Complex sentence: Chavez y Chavez joined Billy the Kid in his twenties, having already having committed a number of small robberies and other crimes, Chavez y Chavez would prove useful to Billy the Kid's gang.
Split into simpler sentences: Chavez y Chavez joined Billy the Kid in his twenties. Already having committed a number of small robberies and other crimes, Chavez y Chavez would prove useful to Billy the Kid's gang.