Co-op Campaigns
A co-operative campaign allows two players to play a full campaign against AI opponents. It is largely similar to a standard multiplayer campaign, with the addition of a shared victory condition: both co-op partners must retain possession of their capital, but required provincial conquests are shared between them. Diplomatic states are also shared between partners, meaning that if one declares war against an AI clan, their partner will also be at war with that AI clan. Similarly, ceasefires and alliances will also be shared. Within a partnership, the field of view on the campaign map will be shared, including line of sight from units and buildings, and the unveiling of the hidden sections of the campaign map.
Unit Sharing
During a co-op campaign, it is possible for players to share troops during their battles. Simply select a unit on the battlefield in the usual way, then click the gift units button in the bottom left-hand corner of the screen. Control of those units will be transferred to your co-op partner for the duration of that battle. This process works both ways so that partners can swap troops back and forth between them as often as they like during a battle. Even if one player has no troops involved, he can still enter the battle and be ready to receive shared units whenever his partner is ready. Once the battle is over, shared units return to the control of the partner who recruited them.
Unit Gifting
In a co-op campaign allies can gift units to each other at will. Once a unit has been gifted it will become a full part of the ally's army, as if recruited by them.