How do two presenters in different locations each share their screen, so onlookers can see both?

Thanks

Only one person can share their desktop at a time: http://office.microsoft.com/en-ca/communicator-help/share-your-desktop-and-programs-in-lync-web-app-HA101913621.aspx

Presenters can switch between shared content quickly.

This isn't possible as far as I am aware. You'll have to share 1 screen at a time. If you try to add a second screen sharing session you will get this message: