In hopes that the OP is still around...

I would sincerely love to have a feature or mode where I can connect a second keyboard to my mac, and then have all input from it go to the second copy of wow I am running.

I run two WOW clients on my MacPro and have a resto Druid and a Hunter. I don't really want to send a lot of keystrokes to both clients at the same time, but I would love to have a second keyboard to send only to the second WOW.

An alternative for use with a single keyboard would be to be able to have multiple states of relaying keys.
One would send all keystrokes to both WOWs.
A second state would send keystrokes to only the main WOW.
A third would send keystrokes only to the background WOW.