You could also get behind the idea that they're novellas instead of novels. That each one can tell their own story in the time they need and not have to fit an entire novel by themselves. It could be more useful than trying to fit two so different things together if they don't have a reason to go together.

Not everything needs its own novel. And it's possible to weave in backstory at the end of one book, depending on how it works for the story. Know the rules, but never be afraid to break them. And always acknowledge that there might be a reason people say these things and listen to the advice.