Get the Goods is the first Greek Quest that has co-op available. It comes roughly around the level 4 mark.
This quest requires you to locate and escort seven Merchant Carts back to the specified point near your town centre.
Once you've explored the map all the merchant carts will be colored red and 'glittery' so they are easy to spot, then build up an army to defeat the bandits protecting them. You only need an army of roughly 7 units to defeat them. And once they're under your control just send them back to the goal mark. A the partner can get these carts too, but only the leader can score them under their control.
Loot treasure works on a "finders keepers" basis rather than a greed/need basis. The first person to select the treasure after the protectors are defeated takes the loot. The other person does not get to see what it was.
And xsoenx is right, it only unlocks for the leader, but should only take 15-20 minutes to do it twice, especially with two people.