Guys, a desync'ed game isn't lost, and can be fixed without too much effort. When a desync occurs, there's a discrepancy in the save game file between you and your teammate, so to fix this you simply need to email the host's file to the second player.
Check this file path:
[your name]/appdata/roaming/The Creative Assembly/Shogun2/save_games_multiplayer
The files won't necessarily always be in order, and number at the end of the file will be one turn off of the one you're looking for. For example, a game on turn 64 will be numbered "65" at the end of the file. Email the file to your friend, have him replace his file with this new one, and you'll both be set to go.
I'd love to see the desyncs stop happening completely, but a friend and I have them happen quite a bit, and this has been a 100% surefire fix to get us back into our campaign with about 5 minutes of effort.