No, you set one pair of outputs as default in Windows (for Winamp, browser audio, etc.), and remove this pair in Ableton. Then both can play separately and simultaneously, but use the Fireface mixer to combine them on your physical outputs (for example AN 1 + 2).
Search the forum for "multiclient" - this has been addressed many (hundreds?) of time already, and also explained in the manual.
This should help with the Fireface Mixer part: http://www.youtube.com/watch?v=splW9OtTBYw