One of the shirts is the show shirt which is worn worn under the band uniform.
The second shirt is called a rehearsal shirt and it is for practices when the band directors want everyone to look the same without wearing the show shirt. For example, these will be used on a Friday morning when the students are practicing at the Warrior Bowl (the football stadium at Westwood). That way the rehearsal shirt gets sweaty and the show shirt can be worn later in the day under the band uniform.