There are other chords in the sheet music that I looked at to try to figure out the voice, but this hasn't been conclusive. At first I thought that maybe the direction of the tie would denote the note's voice, due to the following:
but when I tried this method (first voice for top two notes, second voice for bottom two notes), the notes were in the wrong place:
For reference, here is the surrounding measures.
Followed by the whole note chord, then followed by:
Perhaps this will help with figuring out the voice of the whole notes.
but one of this chord's ties is facing in the wrong direction. Is this perhaps only a visual difference, in that the composer manually switched the curve of the tie to face down?