I'm not sure I totally catch your drift, but I think I know what you mean. In my setup, I can add files and edit my library from either machine and the other machine will "get the updates" because, from either machine I'm using the exact same music files and library file. One thing that I haven't tried (and won't try) that I'm sure would mess it up, is trying to access the library from both machines simultaneously. The reason this works at all is because both copies of itunes are looking at the exact same library file which is pointing to the exact file locations for the music. And when I add music, I add it to the shared drive, which both computer's libraries are pointed to.
Now, obviously both of my machines have local drives that each other cannot see, ie, C:\. If I were on my laptop (the client) and I added music to the library file it uses (M:\iTunes\iTunesLibrary.itl), it WILL add it to the library, and it will work, but ONLY from the laptop. If, after I added the music files from C:\, I went to my desktop (host), and looked for those new files, it would list them, but with an exclamation point because they can't be found. It lists them because it is using the same library file from M:, but it can't find the files because they are pointing to C:\ which, on the desktop, the files are not there! Now, if I copied the files from the laptop's C:\ to the desktop's C:\, it WOULD find them and everything would be just fine, except that any changes to those files (ie tag info) would not carry over from one machine to the other without copying the actual files back and forth. This is why it's nice to keep all the files in a shared drive and point itunes to the shared drive, so that every computer has access to the exact same files.
I hope I didn't talk you in circles too much 😛
Once it's all set up, it's actually not a bad setup. But again I'm not sure if this exact theory can be applied to mac and pc interaction. If the above posters are right, you won't even need to do this 😉
One thing you MIGHT be able to try if all else fails, is to use a sharing medium that both computers can access - a common ground for sharing, like ftp maybe. Maybe you could run an ftp server on one of the machines (the one with your music) that hosts all of your music on either the mac or pc, and then start a library that points to the files on the ftp server. If both computer's itunes libraries point to a library file on the ftp server, and that library file points to music on the ftp server also, then you should have access to your library from either machine similar to the way I do.
Mail.