Select the notes in the portion you want to harmonize from the melody
Copy the notes (Ctrl+C)
For each of the harmony singers:
4a. Paste the copied notes into the harmony singer’s piano track (Ctrl+V)
4b. Adjust the position so they line up with the melody (with all the notes still selected, drag left or right. Hold down the Ctrl key so the notes don’t move up or down as you drag them
Change the melody notes for each harmony singer to harmony notes by selecting each note, and then dragging with the mouse. Hold down the Shift key so the notes only move up or down, not left or right.
When you’re on a voice, you’ll see the notes of the other voices drawn on the piano roll in as thin red lines:
“Two new voices” and “two new tracks” mean the same thing in this context. Each track is an individual voice (I guess unless you do strange things with overlapping groups, but there’s no reason to do that).
Add extra tracks in the arrangement panel, and assign singers to each.
Then follow dcuny’s instructions to copy the melody to the other tracks and adjust the pitch.