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..and by quick I mean a good hour or two. The only win condition would be to defeat the other player.
I feel like the current campaign would have to be altered some to get to the good stuff; increase levels/funds gained by players, more units accessible at the beginning, and decrease AI faction involvement unless reacting to a player's attack or diplomacy agreements. Other things like limiting unit types, army size, removing upkeep, and positioning may be important.
There are many ways a multiplayer focused campaign could be added. If players start within a smaller part of the map then turn times could be limited to only 4 or 5 factions - player's aren't trying to explore or take over the world, just get to each other as quickly as possible. One player could be at their original starting place while the other player starts at a nearby settlement to prepare their attack. Player's could travel to each other which may be fun or both could be moved to an area and have to own a certain settlement at particular turn number in order to win.
It would be difficult to balance this but I feel all the pieces are there. The current multiplayer battle cost system would make a great guideline for armies. Each player has a limited amount of funds they could invest in for their lords, heroes, units, diplomacy, and buildings. It would be kinda like multiplayer except you would be building multiple armies, paying for army upgrades, and then decide how you will attack and defend with that army using the campaign map.