[thakoppno]

Does that require any oddness when it comes to modifier keys like shift, ctl, cmd?

I experimented with double keyboards and thought it required the modifier key to be used on the same keyboard as the other keystroke, which basically raised the bar too high for me.

[harryvederci]

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.