A somewhat workable work around
One way of doing this is using two separate programs to run two different instances of the media you are playing. For example I wanted to listen to a .m3u file streaming from the internet with my USB headphones, while my wife listened to the same drama with headphones. As far as I could tell our 13'' MacBook Pro (spring 2011) did not have any way of using both audio outputs at the same time. After finding no real answer anywhere, I came up with this workaround (will not work for video though, reasons will be obvious)
So, I opened the .m3u file with VLC and chose the headphones as an output from the two listed. Then I opened a copy of that .m3u file in itunes and now my wife was hearing two streams at once, but out of sync. Then I went to system preferences and switched the audio output to the USB and then I was listening to the same stream on itunes through the USB headphones, and my wife was able to listen to the stream through the headphone jack with VLC, and my son stayed asleep.
This will not work for a movie, because one person is going to have the movie start before the other and you can only watch one video feed at a time (unless for some reason you want to see them side by side) and so the soundtrack would be off like a badly dubbed foreign film for one person, or worse if you were trying to use the other output as a monitor for recording purposes.
Hope this helps somebody. It would have been easier just to have the ability to have the sound run to multiple outputs from one source. Oh well.