From a Steam discussion:
Originally Posted by Larian_Octaaf
Hi guys,

When you buy the Digital Collectors Edition you get :
- 2 steam copies of all three games (1 copy for you, 1 copy as a gift), being Original Sin, Beyond Divinity, Divine Divinity
- 1 copy of the DLC

Note to be clear about the DLC ingame:
You do not need this DLC to play the game. It is also not story-related DLC. These are just unique items the host gets one set of when he creates a new game. He can pass this item along to the player in his co-op session if he wants to, but there will always be only one set per co-op session.

So if you and a friend both have the DLC, there will still only be one set of these items in the game session. Only the host controls the creation of the DLC, not the joining player, because this player is joining his story line session.

This would also not make sense lore-wise to have 2 times this unique item in the game world.
Again, if you own the DLC, this will allow you to create games with one set of the items. You can then choose to share these items with anyone joining your session.