I was studying a sheet music for chorus and accompanying piano and have seen a notation that I didn't know how to interpret. There are several parts of the score that shows two simultaneous notes (tones for the same syllable), or a chord, for the voices to execute. Well, to my knowledge it is impossible for the human voice to execute chords... Can someone explain what this means? How do I know which of the notes to perform?
This is actually not uncommon. You perform all of them! In this example, the soprano section splits into two parts. Half of the sopranos sing the top note, and the other half of the sopranos sing the bottom note. At the same time, the tenor section splits into two parts. First and second soprano, first and second tenor. Therefore in this short example there are 6 vocal parts, not 4. In a typical choir, each member will already be assigned to sing the first (higher) part or the second (lower) part within their section whenever the parts are divided. Often the higher and lower parts within each section will even sit on separate rows. Therefore choirs already know how to handle this division of parts.
This is called
divisi (abbr. div.). It is used not only for choirs, but for instrumental music as well and is very common. Just as an example, the 3rd movement from Mozart's 40th symphony in G minor:
The instruction tells the musicians to split into groups, each playing a different line. The lines can move in parallel, but can also be completely different.
Let's look at the score.
The horns staff (in green) is for 2 horns1 and is written in divisi for the 1st horn (top line) and 2nd horn (bottom line). You can see that the parts move in parallel and near the end of the page they merge again. When merging, the editorial options are either the double-stem note as shown above, or the reverse instruction for divisi, called unis. (short for in unison).
The 2 oboes and 2 clarinets2 (in red) have a divisi which is not parallel - the top line holds a note while the bottom line jumps, then they switch - the bottom line holds a 3/4 note while the top line descends. They then merge.
It's also possible for the parts to intersect and cross. One line can start low and rise, while the other starts high and descends, resulting in the 2nd part being above the first.
For choir, the divisi is usually parallel and kept simple, as in your case. In the above instrumentation it is clear for the players who plays which part, but if you have many singers in the choir then your conductor must assign (split) who sings which line in the part.
1Usually you would have 1 horn in G and 1 in Bb, but here Mozart decided that he doesn't need one in Bb.
2The clarinets part is a special addition by Mozart. The lines were taken from the oboe part.