Sticking With One Idea

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It's always been my problem; sticking with one idea for an extended period of time. I knew I would have the same problem with NaNoWriMo but I figured I'd just ignore any new ideas I got and I would have done however I came up with the perfect idea for an extended peice of Chuck fanfiction involving NaNoWriMo. And now I'm already 8000 words deep into my main idea. The problem isn't "should I chuck out the main idea to write my new idea?" because I still want to write my main idea. The problem is "is it inadvisable to attempt to write both?" or I should say "it is inadvisable to attempt to write both, but precisely HOW inadvisable is it?"