Key chords in Keyboard Maestro I'd like to define a Keyboard Maestro macro that I can trigger if I enter a certain key chord. 

As a reminder, key chords are sequences of two hot key presses. Many editors like Emacs, Visual Studio etc. support them and they can vastly expand the set of possible hot key combinations one can use to manage any action.

For example, I could bind Cmd+S Cmd+N to the next song in Spotify (meaning, e.g. S =>  Spotify => N = Next Song). 

Does Keyboard Maestro support chords?
I'd suggest posting this on the Keyboard Maestro Forum in the future – they're very responsive there: 

I don't believe it's explicitly something you can do, but you basically can mimic the behavior. There's two ways. First, you can bind two different macros to the same shortcut. When you trigger it, a palette pops up and you just type the first letter of the macro you want to trigger (or more if they have names that begin with the same letters). For example, if I have two macros triggered by Cmd+S called "high" and "him" then you'd need to type "hig" to trigger the first.

Alternatively, you could use a  "Prompt with List" list and use switch/case. So you would have a list of keywords/letters that are used to set a variable. That variable would determine the path of your switch case. Like so: