A much bigger problem will be the distribution of bans if one of the users commits plagiarism. If you want to share posts with several users, you can just create a shared account which you all have access to.
Is it possible to add on 'View Edit History' the person who edited the thread along with the timestamp? To know who is responsible for a plagiarized content.
...transfer of a thread could be a solution (obviously with a trace).
Thread transfer is also a good thing to carry out useful comments/post.
Would this affect Merits in any way? Say if there are 3 authors, 1 Merit would be given to all or the user gets to choose? Won't it be unfair if the OP only gets the credit while the others contributed their share?
I am thinking if it should be 1author:1merit or 3 authors:merit. If we make it 1:1 there is a high chance that the feature gets abused. Since users can just add people who they know privately and use the feature as a front to obtain merit.
3:1, I don't think merit with decimals is accepted.
After reading your comments, I came to think that it is better to remove the power of the stuff to add users with the OP's thread and remain that power only for the OP. So only OP can add who's the user he want to share the thread with.