Decouple epics and stories on User Story Maps

User story mapping is such a great technique for visually planning out upcoming work, especially across releases. It is a bit frustrating, however, that stories associated to an epic cannot be split across different steps. I routinely have stories within a single epic that span steps, making it impossible to accurately allocate them across columns. It would be super helpful if I had the option to decouple epics and stories in the context of a user story map.