I know this is kind of a weird question for ergonomics because a KB over KB setup would be the opposite of ergonomic.
My situation is that I run 2 computers during the day - I have a KVM that switches when I double tap the Scroll Lock key, but there is no obvious indicator as to which system I'm on and switching over can be a bit slow if you need to do it frequently.
One system is my personal PC, the other is my work PC and while I'm very mindful, I would hate to send an IM to a business associate that is meant for my brother or a friend.
A little 60 or even a 40% KB mounted slightly above and behind my main TKL keyboard would make it easy to use both systems without the risk of getting confused. I know there is software that lets you seamlessly move between systems, but that's not an option.
I suppose a pair of PokerII keyboards might fit decently - does anyone have an elegant solution for two simultaneous keyboards in a limited area?
You can see my keyboard surface here - I have a lot of width, but only about 12" of depth:
Thanks