If on boot it finds two keyboards it can do the same with both.

Mac OS does something like this. If, say, I attach (via either USB or Bluetooth), a presentation remote, I'll get a keyboard identifier alert.

It isn't really anything more than an alert, though, because I can ignore the ID step, and it still works.

I saw a great video years ago (which I haven't been able to locate for years) that went into detail as to how you can basically make a custom usb device arbitrarily malicious. The trick that sounded particularly good was that you can impersonate a usb device that requests a driver that has a known security vulnerability.

Fun times.

The only protection is to restrict what types of devices are allowed to connect. The kernel is not obligated to recognize any device that you attach (though of course most users will expect it to do so!). And of course some host OSes make such restrictions difficult or impossible.