It's long been my desire to looks at the same overlay in Google Earth with one or more colleagues either synchronously or asynchronously. I think I've figured out how to do this.
There is a georeferenced map of London from 1842 from the Rumsey collection. If you are in Google Earth, you can add the overlay from the Rumsey link, but it is clumsy. What happens is that a kmz file is served from the Rumsey site. It appears under Temporary places. Any one who "sees" this layer has to do this. It is not automatically done in a tour (when you do it). For it to happen, you have to instruct a person how to have it happen. This is true of the Placemarks you create as well.
You of course can send the *.KMZ or *.KML files as attachments and instruct the recipient how to activate them. But it is nice to do this in Google Earth. The yellow placemark files are HTML-capable. So you can insert a URL to the kml/kmz files -- if you have a place to serve them.
What I did was use Google Sites from which to do this. The link is associated with the Blog page. The page itself contains a number of KML/KMZ files. Clicking on them will serve them to the Temporary Places area. Clicking on the 1843 map will do this. Clicking on St Lukes Links brings up the Sites page where you can choose which file to activate first.