The attic problem
The common failure is a brilliant past and a present that exists to find it. She inherits a house, opens a trunk, reads letters, and is sad in kitchens. Nothing in her own life can turn, because the book is saving every turn for 1943. Readers feel the imbalance by chapter three. They start skipping the contemporary chapters, which means those chapters should not be there in that form.
A frame is allowed. A frame pretending to be half the novel is not. If one timeline can be summarised as “she discovers the past”, it is a device. Devices can be a prologue, a handful of interludes, or a short present-tense thread. They cannot be every other chapter unless they are a novel too.
Each strand needs want, turn and cost
Apply the same test you would apply to a single book. What does this person want now, in this timeline, that is not “the truth about the other timeline”? Truth-seeking can be part of the want. It cannot be the whole of it. Give the contemporary character a stake that would matter if the letters were never found: a marriage, a child, a job, a debt, a lie she is still telling.
Cost means the discovery changes her life, and her life changes what she does with the discovery. If the past strand could be published alone as a better book, you have your answer. Either deepen the present until it can stand beside it, or stop splitting the reader’s attention.
The braid is a promise
Every time you switch, you promise that the two stories will illuminate each other and eventually touch — by blood, place, object, crime, or a choice that rhymes. If the connection is only thematic (“both women are brave”), the reader will feel managed. Themes are a result, not a hinge.
Do not switch because the chapter count is even. Switch because this strand has just created a question the other can answer without repeating the scene we have already lived. End one on a locked door; open the other on a key, not on a summary of the door.
Rhythm, echo and the danger of the tour
Echo is satisfying when it is structural: the same house, a different use of the room; the same choice, a different price. Echo is wearying when it is decorative: two women look out of two windows and feel hope. One strong rhyme is better than a pattern of mirrors.
Watch the tour. Dual timelines invite research twice — the past explained, then the present walking through a museum of the past. The reader has already been in the room. Do not make them visit it as heritage. If the contemporary character goes to the site, something there must still be able to hurt her.
When to cut the modern strand
Cut it if you are writing it out of obligation to a fashion, a book club shape, or the fear that a straight historical is harder to sell. A single timeline, fully alive, outsells a braid in which half the pages are a corridor. Agents can feel a corridor.
Keep it if the present has its own irreversible choice, and if the ending requires both people — not one person watching the other’s story conclude. The fuse is that double ending: neither strand can pay off alone.
Try this
Summarise each timeline in one sentence of want. If one sentence is only “she uncovers what happened”, that strand is a frame. Give it a want of its own, shrink it to a frame on purpose, or remove it.