Creating Parts for 4 Voices on Closed Staff

I'm trying to create parts for a score so that I can export them separately as a guide for others to learn the respective parts. I put the melodies on a closed staff with soprano and alto on the top staff and tenor and bass on the bottom staff. I've also made sure that the soprano and tenor melodies are under Voice 1 and alto and bass are under Voice 2.

But I'm running into an issue with creating the parts. For example when I try to create the Soprano part, I click on All Parts, rename the new entry from Organ (what it was called by default) to Soprano, then add in the Organ instrument. I only want to include Voice 1 from the top staff but when I try to uncheck all the voices in the bottom staff it doesn't let me. I can never uncheck it. Is there something wrong with the way I set up the score maybe?