This might be exactly what Malby was saying, but as far as I know, server lag would be too big of a hurdle... it would take a different amount of time for each person's voice to get to the server, and a different amount of time to get from the server to every individual listener. So even if the duet was synchronized for some people, it would be seriously out of whack for almost everybody.
I've actually been mulling it over for about a week, and the only way I could see it working is if two people took turns singing verses of the same song, and then it really wouldn't be a true duet with a need for two mics.
However, I would LOVE to be proven wrong... anybody?