Just to be clear - you are both connected to some third person who is host, is that right?
Is it that one of you is keeping up with the host and the other is not? If so, who?
Here's my standard MP checklist for this game:
- make sure all players have opened the required Steam ports in all their active firewalls (each player may have up to 3)
https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711
- all players delete their documents/paradox/ck2/gfx/ folders, and let the game recreate them
- use no mods at all
- all players use the same language in-game
- when anyone gets an OOS error, re-host the game immediately, as it will almost certainly crash soon
- start your multiplayer games and wait half a minute or so before unpausing without any player performing any action at all. Only after the game has been unpaused should players begin doing actions like choosing councils, raising levies etc.