I think the easiest would be a two step process which would involve a “split post” functionality.
Altenatively, when selecting posts, next to (select+replies|select+below|select there would be a mew button “split and select” which would open an editor that would enable the splitting. Probably some custom markings like ~~~ that would work like this:
A part of post that will stay here
~~~
this will be splitted into a new topic (copied there and removed from here)
~~~
this will stay here
If ther ewas a gui way. ie. drawing a rectangle over the part of post to be splitted, it would be perfect but no idea if it is reasonably possible in a browser.