If multiple monitors are present on the machine, you will be asked to choose which monitor you want to share content from when starting a desktop or application share. When mousing over Monitor 1 or Monitor 2 the number of the display will be shown at the top left of the monitor selected. This helps identify which monitor will be shared.
It is also possible to share an application across two monitors. The picture will be compressed to fit on the attendees screen. To do this, first open the application you want to share, then stretch it across both screens. Start an application share and choose the application.