First of all, in your circled example the two note heads have their own stems, so they're not sharing a single stem.

You didn't provide the staff assignment.  Normally, SATB choir use only two staves and the Bass-clef staff would be shared by the Tenor and the Bass.  By convention, the note stem pointing down would be for the Bass and the note stem pointing up would be for the Tenor.  Sometimes they overlap so the upper note whose stem pointing down may be assigned to Bass.

But in your score there are 3 Treble-clef staves: who are they for?  If the Bass-clef staff is for Baritones and Bass (as @phoog suggested), then the notes with the stems pointing up are for the Baritone and the the notes with the stems pointing down would be for the Bass.