Since each story is the work of the author, which means each author owns the characters they create, that would actually be against the copyright law and guidelines, whether or not you plan to release it. It’s like if you’re speeding, whether or not someone catches you, you’re still breaking the law. Plus, someone writes a story in hopes of it getting released, so purposely writing a story that you know isn’t going to get released isn’t exactly what one is looking for when they’re looking for a writing partner.
In case you still have any questions about whether or not characters can be used, just read a bit on this thread:
Crossover Guidelines
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