There is no "Merge" (at least moderators cannot do it) and I've never sensed any merging ever being done.
... ever ...
What I have done for you is that PhilUK's September 2013 resurrection of that 2007 forum thread is now branched away to let it stand on its own. It is as if he'd authored it fresh.
If they have left their user profile default actions set without personalizing them too much, then they will get a response when you attach a new reply to it. You could even consider just pasting a link to your newer post and see what happens. If he responds, the two of you can proceed at greater length and you just leave your # 2576018 post orphaned.
However, if they've disabled that auto-notice feature...
Que Sera, Sera
Thank you. He found my new post and replied to it. I replied to the post you branched with a link to the middle of the discussion, indicating we should continue on the branched post. I also posted a link on the thread with the middle of the discussion, indicating we should continue on the branched post. So, we are now on one post that begins with a question by him, with a link to the orphaned middle of the discussion, in case anybody else is reading it, so they can read all of the pieces in order.
On the OraFAQ forums, we moderators have the ability to do what we call splitting threads, which you call branching here. We also have the ability to merge threads, so I was hoping maybe you have something similar here, but apparently not. When we encounter a hijacked thread, whether new or old, we typically just split/branch it. We have had our own ongoing debate about locking and unlocking. However, we also have a "report to a moderator" feature available on every post that can be used for many things, including requesting unlocking (for which a justification must be provided) or branching/splitting. I don't know if those are features that can be enabled or might be considered here.