If you are using ALSA, you can output to a specific device with xine. Set up an ~/.asoundrc (user only) or /etc/asound.conf (system wide) file that aliases your hardware devices, and then insert that alias in the "device used for stereo output:" box in the xine-engine configuration tab. Alternatively, you can directly enter the alsa device (hw:0,0 or hw:1,0 and so on). If you choose this latter (alternate) method, be aware that ALSA's corresponding plugfoo (e.g. plughw:0,0 or plughw:1,0 and so on) routing through alsa-lib is much preferred.

Here is a sample ~/.asoundrc for a machine that has an onboard Intel AC97 codec and a usb-audio device:

       pcm.intel8x0 {
          type plug
          slave.pcm "hw:0"
       }
      
       ctl.intel8x0 {
          type hw
          card 0
       }
       
       pcm.usb-audio {
          type plug
          slave.pcm "hw:1"
       }
       
       ctl.usb-audio {
          type hw
          card 1
       }

Now enter usb-audio in the xine-engine setup screen and output should go to the usb-audio device (with alsa-lib properly handling sample rate conversion, channels, and so on).

To use a dmixed virtual device, ensure that the "sound card can do mmap" checkbox is unchecked.