What does it mean when there are two chords one on top of the other in a chord chart. An example of what I'm talking about is here.
In this particular case, a jazz lead sheet (but where's the melody?) it's clearly offering an alternative. This is common practice in the 'Real Book' tradition. In bar 15 you're offered a choice of playing Em7♭5 or B♭7.
Very often the choice will be a ii7 - V7 (maybe with a ♭5 or other modification) in place of a simple V7 (as in bars 7-8). But other substitutions are also common.
Yes, a pair of chords, one above the other, can also denote a polychord. They would normally be displayed with a horizontal line between them, like a fraction. But that isn't what's happening here. Thank you for suppling an example, so we didn't have to guess between the various possibilities of what ' two chords one on top of the other' might mean.
It could mean
- optional chords, "this chord works here as well, but a bit differently"
- polychords, stack two chords on top of each other
- first verse vs. second verse, play this on the first round and the other on the second round
In the example you posted, it looks like they're optional chords and you're supposed to try both options to feel how it changes the harmonic feeling, and perhaps learn something about the relationships of the chords. For example the Em7-5 i.e. Gm6/E - A7 motion is something you can try adding anywhere you see a plain A7 chord. The Gm6/E works as a "pre-dominant" to A7, making the feeling of harmonic motion even stronger. In the other example, Em7-5 i.e. Gm6/E vs. Bb7 both work in restoring the song from the short D-majorish detour (D7-G7) back to D minor feel, just a bit differently.
In any case, such notations should be explained by the author. Maybe the "Work Song" you linked to is a part of a wider course, where the notation is explained?
Notation varies, but I was taught that two chords superimposed vertically "is supposed to" indicate a polychord, e.g. two chords at the same time.
It can be easier to express a very complex chord this way, and also gives the performer more clues to go on for voicing.
For this example I would probably play these notes, in this order from low to high:
E Gb Bb D Bb F Ab
That being said, I'd try to find a recording of the song and transcribe the chord myself if that interpretation didn't seem to fit into the context.