There appear to be a number of -quel words formed by back-formation from sequel.
According to this page:
a sequel is a story that takes place later in the same imagined history of a story using many of the same characters and settings
a prequel is a story written or published after a certain other story, but whose events take place earlier in the same imagined history
a paraquel is a story that takes place at the same time as another story with the same or similar characters
an inquel is a story that takes place during a gap in another story’s narrative.
a circumquel is a story that takes place partly before and partly after another story.
when a story is a prequel to one book, but at the same time a sequel to another, we are reading an interquel.
I'm not quite sure which of these would fit the situation you describe, but you have a number of choices.