You're experiencing this for the same reason that you can't github fork
the same project twice from the same account. It has to do with how github actually defines a fork and how it treats it. When you fork ala Github, your personal repo resulting from the fork actually shares the same identifier as the parent project. Trying to fork C from B back into the same account which owns A is thus impossible.
You should use git
commands, it's very common to interact with multiple remotes. In your local repo, add both A and B as remotes (git remote add <alias_B> <url_B>
) then fetch/merge/push changes from one repo to another.