I have successfully set up two sound devices (onboard sound + USB soundcard) so that I can run banshee and rhythmbox and have each play on one device (selected using PA device chooser after opening each program). However upon starting a new song (new stream), both programs return to using the most recently chosen device. They don't stay with their respective devices.

Is there any way to set different fallback devices for separate programs in PA, or keep them from falling back to to a single device?