We have the optional application paprefs that allows to install a virtual output device to pulseaudio. This enables simultaneous output to all attached sound cards/devices:
The additionally created audio output device for simultaneous output may be selected in the “Output” tab from pulseaudio sound preferences menu:
From the command line we can also load the device for simultaneous output without having to run paprefs by
pacmd load-module module-combine-sink # pacmd load-module module-combine # for PulseAudio < 1.0
Put this line (without pacmd) in your
/etc/pulse/default.pa to load this device at startup.