1) Browse to a forum (this will be your first tab)
2) Open new tab (this will be your second tab)
3) Browse to a forum and click a thread title
4) Click Thread Tools
5) Click Select for Thread Moderation
6) Click your first tab
7) Select a thread for moderation
Notice how only one thread is being shown as "Selected Threads: 1".
Another option would be to allow pasting a URL of the thread to be merged. So instead of clicking the Thread Tools and selecting the thread, the admin would copy the URL, then select another thread from a forum-view and then invoke the Merge Thread option where an input box would allow pasting the URL of the other thread to be merged.