[0]: https://superuser.com/questions/707225/how-can-i-combine-the...
Mate, you have no idea how frustrating this has been for 4+ years.
It used to work fine, and then Apple did something that broke it... and this tool lets me push Shift / Option on one keyboard and have that reflected with what signal my mouse keys send. Thank you!
I haven't got a clue why this works on every PC, or Linux computer, and it used to work on Mac, but then all of a sudden it didn't work on Mac... just strikes me as an absurd anti-accessibility Mac choice. But hey, this fixed it!
(It was a bit of a beast to install, not gonna lie, took about 8 restarts, and had some weird hang as I was installing drivers and all the keyboards stopped working... and I was a bit scared but I held down power to cycle it off... and luckily that key worked... and when it came back on it started working. Ha.)