I expected those kinds of issues, but it actually works well!

So I can just do left-shift on the left keyboard, any right-side letter on the right keyboard, and it will be uppercase. Same with ctrl/alt, etc.

I can't promise you won't run into something that doesn't work, but I haven't found such a thing. On macOS my Karabiner keybindings also work without needing modifications to the config or whatever.