I want to make the post merger to be:
Double Post:
____________________________________________________
Message
How would I do that? I already tried in the acp. And had no success.
Try changing what the double post separator is.
I did

I put
<br>
Double post:<br>
But this isn't what I wanted.

It doesn't even appear like that.
It probably doesn't accept HTML then. No reason why it would tbh.
I suggest creating a [br] custom MyCode which is replaced with <br />
Then you can put in the setting:
[br][br]Double Post:[br]
ohh, bad results.
It came out replacing tons of things with spaces therefore messing up every post on the forum.
how do you make the post merger work at my forums it doesn't work
Well you would go to AdminCP - Settings - Posts
But the only thing that is there is the
Which is pretty ugly.

You could just leave the post separator field empty, then it'll just look like 1 post.