I suppose, you could do it by writing some code that sends all post requests from one site to the other site, for both sites. You might end up with heavy duplication (every post will be saved at both sites).
Perhaps you'd be better off using a third (data) server or picking a primary (site + data) server, and holding only short references to posts on the other (site) server(s). Then, when a site displays a post, it requests the appropriate information from the primary/data server.
It'd require some careful thought; there's a number of technologies you could use to setup communication between the servers, and a number of ways you could store the information.
You might suffer a load-time disadvantage if you're having to connect to a remote location /read from a remote location every time you have to serve a request; not to mention a much bigger security concern.
By far, the most efficient way is to park multiple domains into one big server (all the data is on the same physical computer so sharing is easy; but there appears to be more than one site), or to use a number of servers in the same physical location with a protected local connection between them.
As you consider such things, it gets more and more costly...
iFrames could work =P
