Would triggering a conflict palette with another hotkey, which would act as the "leader key", achieve a similar effect?
If so adapting this idea, still untested beyond Mojave, might put you closer to your goal.
here's a test macro to run.
It has two "conflict" macros, both using the F7 trigger key.
A third macro, named Leader Key, with the trigger F8, calls two AppleScripts that simulate the F7 keystroke .
On my Mojave run system, a conflict palette with the 2 macros, named A macro and B macro will appear. Pressing "a" key triggers A macro to play Submarine sound. Pressing "b" triggers B macro to play Blow sound.
Simulated Leader Key.kmmacros (5.6 KB)