|You can only merge copies of the same shared workbook, not 2 different workbooks that have been shared.|
The theory is that you can make multiple copies of the same shared workbook, distribute it to multiple users and then bring them all back together to see what changes have been made by each user.
So in order to use the Compare and Merge feature, it sounds like you would need to have a workbook that contains both Week One and Week Two, which you would Share and then Copy.
Then you would make changes to one of them and use "Compare and Merge" to compare it to the other.
From some reason I'm guessing that that is not what you want to do.
Why not give us a few more details as to what you are trying to compare and we'll see if we can help.