Airplay: stream video and audio components to different Airplay target devices
It just isn't feasible at this time. Video and audio need to stay perfectly in sync for your brain to not notice, and splitting up the streams of data to two different destinations creates a time lag that, at this time, is impossible to sync up.  The reason why it works at all going to one device is that the destination spools up both streams and starts them both at the same time.