I can positively say that what you want to do is indeed possible. I achieved the specified scenario by first
sudo cat /dev/input/event(0,1,2..)
while typing on each keyboard to ensure that they were recognized as separate devices. Then I simply applied trial-and-error, experimenting with
setxkbmap -device (1-x) us.
I discovered that
setxkbmap -device 1 us configured both (all) keyboards, while
setxkbmap -device 6 us only affected the secondary. Of relevance is perhaps that they both were connected via PS/2.
This of course is not guaranteed to work for you, and doesn't even classify as a proper answer, but it confirms that it's at all possible. I don't know according to which scheme the individual keyboards are named in
/dev/input, but I'm sure that the names can be specified in udev, making it possible to put the appropriate
setxkbmap command in
/etc/rc.local or similar.