Authors, Writers, Publishers, and Book Readers
Well, firstly I'd say you are incredibly lucky to not only be writing the greatest story ever, but to also have another great idea! I struggle with coming up with one in the first place! lol... but I know what you mean and it does seem that once you open the playpen and let out your brain it runs riot! At least mine does... for this reason I have a million notebooks filled with random thoughts and pieces of dialogue that don't seem to fit into what I'm writing.
What I do is if I get any ideas that don't seem like they fit into the story I'm currently writing, like I said above I have a that I write them in, then usually as I get further into my story, or even after I've finished that one and moved on to the next I have a look through my notebook and often find that I can fit it somewhere.