I've not had any experience myself (the closest I've done is multiple streams to multiple sound card devices).
How does the ALC888 drivers display itself to the operating system - it is a single 10 channel device, or does it have separate deviceIDs for the separate stereo lines?
If it has seperate deviceIDs then WaveOut can be supplied the deviceID.
If the driver exposes the 2 separate lines as just an extra set of channels on the same deviceID, one nasty hack way would be to pass the PCM data with lots of channels, only providing data for the relevant channels.
Or a cleaner way is that various audio frameworks (PortAudio, XAudio2 / DirectSound) let you specify a channel mask and mapping.