Having to distinct hard disks for each operating is the optimal solution, so go for that.
Install Ubuntu on the other drive, but avoid during installation to overwrite the GRUB boot options, unless the installer detects the Fedora system and offers to include in the new GRUB setup for booting. The boot settings should have both Fedora and Ubuntu in the end.
Read some GRUB documentation for tampering with the boot settings by hand.
Last edited by Tinkster; 10-30-2010 at 06:17 PM.