@EricShulman asked, how to split a thread
I usually start at the top of the thread. Right above the “right” navigation widget there should be a “wrench” symbol for admin users. It contains the “Select Posts” menu item.
If clicked it shows 3 buttons above every post.
It is important to manually check if you got all the related posts, since not all users use the “reply” button. So “Select +replies” does not “catch” all the related posts.
Be aware, that the “cancel selecting” button is only a link. So it’s easy to miss that one 
Once you are done, you select “move to”.
which then shows, a dialogue, which you can use to create a new thread.
It is also possible to move single posts to an existing thread.
Hope that helps