I first experimented with parallel story structure when I wrote Plague of Shadows for Paizo’s Pathfinder series. In that book, I’d occasionally flash back in time fifteen years for vignettes that slowly revealed background relationships illuminating why the characters acted toward each other the way they did in the present.
It wasn’t a NEW technique, for I’ve seen it done elsewhere, and you probably have as well (especially in Scott Lynch’s Gentleman Bastard books). But it was new to me.
In my new series I’m attempting to use it in a far more complex manner, for approximately half of each book (thought not all in one chunk). Characters who are dead or missing in the present are point of view characters in the past, and they are searching for explanation of mysteries that we see characters in the present discovering (all while dealing with the aftermath of actions taken by both themselves and these other POV characters long ago).