I’m contemplating writing a dual narrative, because after much thought and research, I think it’s the most interesting/effective way to tell my story. All the dual narratives I’ve seen alternate narrators by the chapter. I’m wondering if it HAS to be like that.. as in, if I didn’t stick to a pattern or chose a different one day (say, a-b-b-a) would publishers turn it away? Or does it not really matter so long as it’s done well?
Chapters are metatext
There is no obligation to label any division of book a 'chapter'. The narrative can change direction without the label, and within the label.
Put another way, you could write CHAPTER ONE in block letters on page 263, and never refer to it again.
There is no hard definition for what a chapter must be, so superficial answer:
Do dual narratives have to alternate every chapter?
What chapters imply
You will be able to set your own rules for what 'chapter' means in your story, but I think the generally accepted idea is that it is a 'natural' break in the narrative.
Narrative reasons for chapter breaks are similar to 'acts' in Hollywood's 3-Act structure: each 'chapter' has a direction, a goal, a point to make. The narrative will 'break' on a poignant moment, then shift directions/goals/POV with a new 'chapter'.
As readers come to expect and appreciate the pattern, a chapter break signals something profound has happened. It can influence how the narrative is perceived, punctuating the story.
What patterns imply
Any pattern you establish is imposed on the reader, not yourself. Don't limit your narrative choices (POV) to preserve an arbitrary (metatext) pattern.
The structure at the start of your story may not be appropriate at the end.
The pacing will change. Your protagonists will eventually share scenes and the POV will need to shift as needed within the scene.
Establishing a pattern only to break it at a dramatic point in the story can be effective, too.
If one of your protagonists is missing or thought to be dead, and he doesn't take his turn providing POV, that could be an fun interplay of the metatext structure, narrative, and reader expectations.