How do I get Stereo + 5.1 for YouTube videos?
It's not very clear what they mean. If you look further down on the same page it says it accepts 5.1 @ 512 kbps.

Recommended audio bitrates for uploads

Mono    128 kbps
Stereo  384 kbps
5.1     512 kbps


YouTube doesn't currently support 5.1 playback. If you upload a 5.1 audio track it gets converted to stereo.