My original post to the xorg list (with responses by Peter Hutterer) was:

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>       One thing I just noticed is that the root window property for XKB only 
> contains the information for one keyboard.  When I run setxkbmap -print, the 
> device parameter doesn't make it read the settings for that device, it simply 
> relies on this property, which is presumably the most recent one set:
> 
> _XKB_RULES_NAMES(STRING) = "evdev", "pc105+inet", "ang,el", "", \
>       "grp:switch,grp:scroll_toggle,grp_led:scroll"
> 
>       Should that somehow represent multiple keyboards?

yes, it should. but can't at the same time. basically this property has been
around for long enough that we can't change it. for the multi-keyboard case we
should just introduce a new property. patches appreciated of course.

note that you will need to modify setxkbmap as well, xkbcomp already takes a
device parameter.
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HTH