I have done something like this by adding another InfoWindow as child to the map such as this:
private var anotherInfoWindow: InfoWindow;
// substantiate the infowindow
anotherInfoWindow : InfoWindow = new InfowWindow (map);
// add the infowindow to the map as a child
map.addChild(anotherInfoWindow);
// add handler to handle closing of the infowindow
anotherInfoWindow.addEventListener(Event.CLOSE, anotherInfoWindowCloseEventHandler);