I think using BASSmix will be the easiest way to do that. To keep the streams separate (eg. one on the left and the other on the right), you can use the SPEAKER flags...
BASS_Mixer_StreamAddChannel(mixer, streamA, BASS_SPEAKER_FRONTLEFT); // put the 1st source on the left
BASS_Mixer_StreamAddChannel(mixer, streamB, BASS_SPEAKER_FRONTRIGHT); // put the 2nd source on the right