I think what he's trying to say is a sort of 'alliance' between countries where they both invade another country, and split the land in accordance with the agreement made before the conflict.
Something along those lines anyway.
If we have a diplomacy system like that of Victoria, this might not be needed, as you can demand/give provinces accordingly (though, in Victoria I can't remember being able to achieve provinces for my allies (but why would I want to

))