Originally Posted by linus72's Signature
@OP; Clarify: Any new installation of GRUB should detect both OS'es. There should be no need to re-install anything, since it is essentially GRUB's 'job' to boot any OS...
Possible Answer: If you can boot into either OS, install GRUB from there. No additional steps will be needed since the package manager of both Fedora and Ubuntu should automate the process.