Hmm... so from the sounds of things it'd go something like this:
Wife and I make our characters which will be completely separate at first. Then at lvl 2 we can get to a hub city and then meet up.
At this point we can do each other's story quests together.
Wander around the world doing hearts, dynamic events, and the like together
Then when we feel we're high enough level mosey on over to the dungeon and grab three random other people there and enjoy a simple story mode version of the dungeon before moving on to another area.
Does that about sum it up?