We tried a few missions from the beginning with a couple of friends, and it seemed to work ok. Some small problems here and there, such as only the host seeing slides shown in a briefing, but overall both the game and the campaign worked really well. There isn't really a separate coop campaign, it's the same campaign and same saves in both coop and sp. What this means in practice is you can continue an already started sp-campaign in coop, then move back to sp for a few missions, then coop again, etc...