Books with dual time lines - a thread set in the present - or near present - and one set in the past, are popular with readers. Two stories that link together, where the past has consequences in the present. Juggling two narratives can be complicated, but very satisfying to achieve. And as I said. readers like them. There is one issue though that always comes up when a group of authors who write this kind of book get together - how to make the connection.
The classic is the discovery of an old diary. Or a cache of letters. A photo album might work, and possibly a portrait or a painting - but that is more likely to be used in a time slip or time travel when the protagonist ends up physically in another time.
The problem with classic is that it can feel like cliche. And no author likes to feel that they are offering their audience cliches. A lot of discussion goes into trying to think up something new, but it is hard, maybe impossible.
And really, does it matter?
Readers are happy to find books that offer their favourite tropes. Enemies to lovers, marriage of convenience and so on. It is a positive choice. So if you read dual time line are the letters or the diary simply part of the 'trope' of the genre? Are they even part of what makes the style attractive to a reader? Are writers worrying too much about feeling cliched?
I'm currently debating the question because the book I am working on, while not strictly a dual time line does have an element of the past influencing the present and I have to make the connection. I have found a way, I'm not saying any more, because spoilers but it still depends on a paper record - not that far away from the letters and the diary. But I really can't think of a way around that.