I am currently working on a project that is a spinoff (fork) from a framework I have been working on.
This project is intended to be pretty generic, but now I need to fork the codebase again for a client of mine.
At this moment, I have created a custom branch for my client, but I'd rather have a standalone repository for this.
I contacted GitHub support about this 'issue' and they confirmed this is not possible. Also, it is very unlikely that such a feature will be implemented in the future.