In case you want the merge to look good, here's my take on what you could do:
- Add the horizontal line BBCode to the forum, which should look like something like this:
[hr][/hr] (it's also useful for normal posts for some cases, look it up);
- Make the "merged" text more discrete below the horizontal line, with a smaller size, like so:
I don't think it needs to be very fancy, to not take away too much attention from the posts themselves, so it should be clear, but at the same time as discrete and simple as possible, imho.
If you can use HTML and not only BBCode, you can do the horizontal line with the HTML tag directly:
In the end it would look like something similar to this: