Does YouTube allow multiple soundtracks
When uploading to YouTube: no (for now...)
Your audio needs to be premixed into one stereo (or mono) channel.

When using YouTube's online video editor: yes, but you are limited to voice-over and music tracks

For more info:
http://support.google.com/youtube/bin/answer.py?hl=en&amp;answer=183851
(see point 4. adding audio)