Main character girl on side a finds government secret somehow. First idea I had for this was that the character has a hobby of piecing together old shredded documents (where would she get the documents; why is this her hobby, if this is the discovery mechanism; what's her name). How else could she find said secret? How is the rift opened? It has to be relatively easy, but not so simple that it would happen unintentionally. The secret could start between just the two girls, one from each side. Side b girl could have a brother that also gets involved in book 1. Conflicts can include trying to keep it a secret, trying to learn more about it, personal conflicts, and environmental conflicts on both sides that start to expose the cultural differences between them, especially in the second half of the book. I drew a timeline last night that put rift discovery around 1970, some hypothetical important event at 2020 (not sure why this would be a thing, but it was an idea I had for some reason, balance I think), and the story taking place circa 2070.

The backstory could be that the information was leaked unintentionally, probably something concrete and highly believeable, like a series of videos. The leak could even be from an unintentional misinterpretation of some memo by a junior staffer who just assumed that the language used was directing him to release it, even though he's unsure, he could do it anyway in a typically confident fashion only to later be informed of the miscommunication. (He was probably fired for it. Who knows? It's not really important.) Maybe the 2020 events could be something of a repeat opening (intiation???/second split???/third side???) of the rift, where something else important is learned / revealed.

Perhaps the first direct interfaces to control your own biology could be coming out in side b, and they could be focused on emotional regulation and sharing. Whereas side a would probably have further developed that technology and developed it earlier but focusing on strengthening the elite class's connection to hardware/software compute services and to additional hardware memory.