Can you be more specific what you think can't be done?

I just tried it, and I can without problem open a PR from a fork against an arbitrary branch of an arbitrary fork (or main repository).

You can't open a PR in the parent repository against a branch in the fork. You can create that PR inside the forked repository, but that doesn't allow the maintainers of the parent repository to do anything with it. So if you've got two stacked branch in a fork, you can't open two PRs for them in the manner GP described.

but if everyone works at the same company they can create the branches in the same repo