Ok, I have this problem, I'll try to explain it :
I used to work on several novel(a)s at the same time, more a curse (not able to focus) than an aptitude.
Regulary I find that an idea, a subplot, a paragraph (or just a good sentence sometimes) could fit in two stories. It works well on each, offering good possibilities of developing the charachters, storylines... etc.
And from there comes the dilemma : I have to make a choice, so I spend several days or weeks, changing my mind, hesitating, making me feel like I'm Buridan's ass !
I don't expect a miraculous answer, but if you already had the same problem, It would be good to share experience.
Thanks.
Robert Gill, It's not really what I wanted to talk about, but to stay on your subject, I used to think this way, about ideas that you may find so great that you would just write a new novel to put it in, or ideas that had to be stored, waiting for a better moment, a better story to be used, but it doesn't really work for me (It took me time to admit it) cause this "better" story could never appear, or take too much time to appear, so now, I welcome everything comes, whatever it is, and if I can, I try to place it in my "work in progress", It's less calculation, more peace of mind, some kind of Carpe Diem ! )
Sometimes I find it useful to write the idea into an "Ideas file" and leave it alone for a while. If the story really can't develop without it, I try to drop it in at a place which feels natural for the characters rather than at the first or latest opportunity. If I don't think the story or characters are ready for it yet, I usually hold off and tweak the story into a direction where it will eventually feel natural. Perhaps revealing the information gradually over several chapters may be better than delivering it in a single line? Or have the characters in conflict with one another over the entangled areas until one is forced to back down or who cedes control over that area? I don't know if anything here is of use, but it may be worth looking at the event in a different delivery style.
Just dig one up and move it further away ;)