I vaguely recall the software toggles you've mentioned. I'm pretty sure I've seen them in Win/Control Panel. But the stumbling block may be here.
The two machines will need to be connected to a network, of course,

The Internet for that purpose is out of the question.
I wouldn't mind having a hard wire connection between them. But that means buying hardware. And these cables aren't cheap.
Swapping keyboards is a tad dreary. But it's conceptually obvious, and technically simple.

None the less, it's good to know it's an option.
Right now I use a Western Digital Passport HDD, a slim little HDD ~150 gigs capacity (easily fits in a shirt pocket). It uses the FAT format standard, which is both Windows and Linux compatible. It has no additional power requirements, obtaining its power via the USB connection directly. (it's also 20 gigs higher capacity than the internal HDD of the Apple Mac mini)
I just swap it back and forth manually via the USB ports.
That's not elegant. But it's adequate.

BTW, this little HDD may be on the verge of crashing. I'm not sure. I used to use it for nearly everything. It's even got several vintage movies on it.
But several little drives are great, not for capacity, but for redundancy.
And backing up the data on it is a breeze with Windows. I just write a batch file, drop the icon on my desktop, and whenever I want to back up whatever, I just click that icon, and the computer does the rest. And it'll back up only the files, & or folders I specify, to as many other drives/folders as I want.

One click!
Sweet!

"when the bigots of this world have been privileged for as long as they have, to them equality feels like discrimination." shiftless2