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Unfortunately, on OSX - cross-modifier keys did not work. If I weren't so cheap - the perfect arrangement would actually be for the matching-leftmost keyboard to not have arrows/numpad so that the right doesn't have to be raised - but I can forgo a lot in the name of 'free'. This arrangement allows for either hand to use either keyboard, allows me to "cross-over" without having to reach across a barrier for "middle" letters like TYU, maintains a numpad (on the far-right), and creates a quite comfortable ergonomic situation.
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This allows my hands (when sitting on home-row keys) to be squarely aligned with the armrests of most office chairs. I took a spare heat-sink that was about the right height and raised the right-most keyboard so that it sits over the arrow/numpad of the left-most keyboard. I've seen many other people do similar things. So, for Linux - I ended up just plugging in two cheap Dell keyboards and using them together. These "cross-over" habits make a split-keyboard unrealistic for me to use. Often if it's a quick sentence I'll type it 100% with my left hand if my right hand is on the mouse/trackpad, etc. Unfortunately I realized I often do bad things like typing 'Y' with my left hand or other cross-overs. Once upon a time I got really into ergonomics and thought I wanted a split-keyboard. So now only using Karabiner-Elements, my settings are: I installed Karabiner-Elements and with no modifications other than running it got-back my "default" function from the original Karabiner of having the modifiers work again. While looking about that I found that Seil has since been deprecated and its git site now points you to Karabiner-Elements.
#Karabiner elements modifer key update
I didn't know a good place to shove this - but now in 2020 (my previous update to this page was from Jan 2016) and running MacOS Catalina 10.15.5 my Karabiner settings below ceased to allow me to use a modified (e.g., "shift") on my left keyboard and get a capital key from pressing the other key ("m") on my right keyboard.
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