Great idea. So you have hands wide apart and canted outwards?

Thanks!

I keep my hands at shoulder-width and put them on the desk in a natural-feeling position. Wherever they land, that's where I put the keyboards.

Not sure if there is a better appraoch to place them in the best ergonomic position, but my wrist injury has gotten much better lately. That may just be a coincidence, but it feels like it's at least safe to say what I'm doing isn't making the injury worse.

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.

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.

I did something similar with two Apple Bluetooth keyboards. It worked perfectly for about a year while I waited for my UHK to arrive.

Ha, that is damn clever