I've thought a bit more about the rift mechanic. I think maybe it can get stronger over time. In the b side, it's common knowledge and everyone learns about it in school and as new things become possible, everyone learns about them pretty quickly. But on the a side, it is basically a government secret held by each nation that knows of the rift. The rift is present everywhere, but openings are made only in one direction. You can only open a rift that lets things in, not one that forces things out. It was discovered on both sides at the same time by a research facility around 1970. Some kids discover it on the a side, and the kids are probably already in high school; they happen to interact initially with kids on the b side and convince them not to tell others about it right away. Maybe even though everyone on side b knows about rifts, it fell out of practice to leave them open because there was basically never a time when people from side a had them open in the same place at the same time. And, the family from side b that still maintains an open rift at all times because the mom or dad is just kind of weird like that.

Maybe some government of side a can be involved secretly at first, just monitering the interactions, and then they can come up with a plan to steal technology from side b without giving away anything from side a. How would the government do this without the kids knowing? It could be traditional survailence (or like the level of survailence that people think happens today, and which probably does to some extent - like conspiracy theory level survailence). They might even coerce the parents to help them spy.

I'm not really sure what the plot can be that uses the rift mechanic, and actually I don't care if the plot is important to the rift mechanic. The plot would, however, need to contribute to sharing technology that saves side a from pressures that they broght upon themselves. Maybe the long term goal of both the kids and to a much lesser extent the side a government is to increase the ability to interact through the rift. I'm not really sure I like that idea about death affecting people across the rift - seems unfoundable-ish.