You can emulate only one device at a time.

If you double-click one device in a current selection, an icon appears next to the double-clicked device. The Emulator tab plays the content from the beginning.

If you double-click a device other than a current selection, Adobe Device Central removes the selection and selects that device instead. The active device icon  appears next to the new selected device, which is loaded into the Emulator tab. The Emulator tab plays the content from the beginning.