The Server.MapPath
method is used to get the physical path of an url. It has nothing to do with creating an URL to a file. Even if you can reach the physical files of one web site from the server code of another, that doesn't mean that you can specify an url to do that.
You can't create a relative path from one domain to another. Even if they happen to be on the same server, there is still no common parent for the domains that you can use to make a relative path from one to another.
Each domain name just points to an IP address where you reach the server that handles the request, and there is no relation between domain names at all. Even if two domain names points to the same IP address, that doesn't mean that they need to be on the same server, and if two domain names point to different IP addresses they can still be handled by the same server.