OSX, different keyboards with different input methods possible?
Based on Tom Gewecke's input, I've done the following:

Installed Karabiner from: https://pqrs.org/osx/karabiner/index.html.en
(note that Karabiner 10.22.0 does not work with macOS 10.12.x at the time of this reply)

Installed this mapping into private.xml from: https://gist.github.com/ylorn/43b589daf6d621a7fc47 
(I've changed it to left shift key and control space to match my preferred international input keybindings)

This approach does not allow me to set default language inputs per keyboard, but does allow me to more quickly switch between them.  An acceptable compromise until I can find something better.

After asking Karabiner to reload the xml files, I can now tap the left shift key to switch inputs quickly.

--

In macOS 10.12, you can achieve the same functionality with the caps log key, without Karabiner, by enabling the "Use the Caps Lock key to switch to and from US"