In the flashback, he thinks, ' Heretical Dark and Sublime Light,' and then says ' Some day I'm going to slash 'em both.' It's assumed that by ' 'em,' Jack is saying he wants to slash Heretical Dark and Sublime Light. Jack doesn't say directly that he wished to cut their magic.
Moreover, Jack's words in the past and present are ambiguous enough that it creates the impression that he may have not even achieved anything. All of this happens to Jack the Ripper, and it's even sadder when considering the fact that his slash doesn't kill their opponent. In manga, a flashback where a hero shares their dreams before dying in pursuit of said dream makes it very likely that the hero in question died.
