Well, my wife and I started playing the game over the last few days, so I'm going to leave some notes here for anyone else in a similar situation.
However, I'm going to say that no, it is not really viable to play the game completely as co-op.
Here are a few things we've come across already, being about 10 hours in:
- As KatieK said, it takes a while before you can even connect. 10 hours was probably a really high estimation, but I'd say to expect something in the 3-5 hour range before you can play co-op at all, depending how completely you try to explore the early areas.
- Firstly, and probably most importantly, when playing in co-op, only the host player will progress through the story at all. Even if you join up at the exact same point in the story and go fight the next story boss together, the guest player will not receive credit for it. So if you ever go back to playing solo, you'll be at exactly the same point that you were when you joined another person's game.
- The game has (at least?) two types of treasure chests, red ones and blue ones. The blue ones have semi-random contents and refill themselves after a while. The red ones are pre-defined, but only the host player can open red chests, and they will get the contents. This means that if there are any items you want in red chests, a visiting player will not be able to get them.
- Only the host player will be given any quests from NPCs, the guest player gets completely different non-quest dialogue if they speak to the same NPC. This means that only the host player can complete quests and receive rewards from them.
- There seems to be no system at all for giving equipment to other human players. You can transfer items, but not equipment. This means that if one player (likely the host, due to other issues) receives a good piece of equipment that they want to give to a guest player, there's no way to do so.
- A guest player can not bring any of their other party members across with them. So, much like story progression, if you ever go back to solo play you're going to be way behind, with your party members completely un-leveled compared to your main character.