Two plotlines that don't have much to do together
Question: I plan to write a pretty long sci-fi fiction story.
I spent a lot of years writing but (mostly rewriting) this story, wrote about 400 pages of it and I know how it will end, but now I realize that my story has two plots that don't have much to do together.
To sum up, I'm writing a story about a kid who wants people to count on him, but then a pandemic happens. His parents put him to safety by sending him on another planet, and while I develop how people deal with the pandemic, I also write about the kid's journey on becoming someone better, which has nothing to do with the pandemic. I'm not finished yet, but I realized that the two stories never merge again till the end of both stories.
I heard that an author should be able to sum up his story in one sentence, and this is the fact that I couldn't do it that alarmed me. With that story, my plan was to question the reader about why people deal violently with things that happen to them, but that is supposed to be the hidden message so I feel like even if it links the stories together, it doesn't count.
I also feel like I can't dissociate the two plotlines to make two different stories though, because everything happens in the same universe, so I'm using some of the plot A events to explain some of the plot B events and vice versa, so now I'm stuck and I can't write anymore.
What should I do?
Just in case it might be important to mention it, I intend this story to be a manga, and I feel that most of the novel writing tips are useful for mangakas, so that is why I'm posting my question here.
The way to connect the two plotlines (the external story of the pandemic and the internal story of the kid's growth) is to have the kid make a choice at the crisis of the story that causes the ending to occur.
The lesson the kid learns on his journey that makes him a better person needs to be what causes him to make the right choice at the crisis. That choice might save the world or it might just save the kid (depending on whether you want a happy ending or just a personal victory ending).
To say that another way... Your character should be someone who is uniquely suited to bring about the ending of the story. And the reason he is uniquely suited is that he learns something on his journey that the people back home don't understand -- which is why they can't solve the problem on their own. Perhaps he chooses to react nonviolently to something, in a way no one on earth would, and that changes everything. (That's just a suggestion.) Either way, he should make a choice that his old self, the person he was before he went on the journey, would not have made.
All the plot events on earth should build to the crisis, which will be the point where it appears the problem will not be solved and disaster is certain. The main character's choice then should allow the situation to reverse so the ending can happen.
If you want to summarize this story in a sentence, you can use a formula like this (just replace what's in the brackets with something specific)...
"When a pandemic threatens to destroy humanity, a kid with (a unique trait, insight, etc.) manages to avert disaster by (whatever his choice will be), resulting in (whatever ending you want)."
Best of luck.