Playing/silencing joint staves

In some of my pieces, I have vocal parts with two staves. For example, the soprano part has two staves but they are both labeled as part of the soprano "instrument." Because of this, when I go to play the soprano part, it plays both staves at the same time. However, I need it to play them separately. When I go to the mixer, I can only silence both soprano parts as a whole, how do I separate the two lines so I am able to silence them and play one staff at a time?