I work for ACE team, and want to cross-post from http://blogs.msdn.com/esiu to http://blogs.msdn.com/ace_team. Community Server supports MetaWeblog API, but I am not able to figure out how to configure cross-posting. After a few tries, I am able to cross-post now.
Community Server asks for several things
URL: http://blogs.msdn.com/blogs/metablog.ashx. Don't use http://blogs.msdn.com/<appkey>, and in my case, http://blogs.msdn.com/ace_team.
AppKey: target blogid, in my case, ace_team
Username: target blog login
Password: target blog password
Message to add: A header message on each cross post
Last not but least, you need to enable cross-posting in Global Settings -> Default Post Settings -> Enable Cross-Posting.
Hopefully, this will save you time as I spent numerous research hours figuring this out. I also apologize for polluting the blog atmosphere by a few phantom test messages while testing various configuration settings.