What does choir music with only two staves both with treble clefs in the vocal parts (often labelled "Voice 1" and "Voice 2") mean?
Voice parts that have a lot of rhythmically independent movement need to have their own staves to make the music easier to read. Each will get labeled as a "voice", so two such staves will be Voice 1 and Voice 2. Some in the group will sing the Voice 1 part, others will sing the Voice 2 part. Often, but not always, the Voice 1 part is higher than the Voice 2 part. Sometimes they are in the same general range but the rhythms are so independent that the parts must be separated.
Sometimes these two-voice pieces will say they are suitable for mixed chorus. That means that one part, or both, can be sung by either men or women. Tenor parts are often written in treble clef but sung down an octave, so these parts can often be turned into a part for men. A children's chorus or women's chorus would sing both parts as written in the treble clef. Simply labeling the parts as Voice 1 and Voice 2 makes the voicing more flexible, rather than saying Soprano and Alto, which would specify a specific range and voice part for each stave.