I have two sound cards, too. One is integrated in the mainboard and the other one is a USB device with an amplifier for my record player. With PulseAudio I am able to switch over to a different sound device for output.
I did the following in order to install PulseAudio.
Code:
sudo apt-get update
sudo apt-get -t testing install pulseaudio paman pavucontrol paprefs
Afterwards I created a file /etc/asound.conf with the following configuration as described on the page http://pulseaudio.org/wiki/PerfectSetup:
Code:
pcm.pulse {
    type pulse
}
ctl.pulse {
    type pulse
}
pcm.!default {
    type pulse
}
ctl.!default {
    type pulse
}
I rebooted the system. In the menu "Sound & Video" I found the "PulseAudio Volume Control" which I use to select the output device.