*** Howto Multiboot Fedora ***
Somewhere in recent times grub2 has been fixed. The horrible mistakes that I reported in https://bugzilla.redhat.com/show_bug.cgi?id=837430 on 03 Jul 2012 for F17 are fixed. The program that constructs /boot/grub2/grub.cfg now is once again capable of finding bootable kernels in multiple root partitions (or even a Windows partition, if your level of retardation requires such) and constructing "menuentries" that work correctly with their respective root partitions and home partitions. Therefore, the complicated deviations from the standard installation sequence previously documented here are no longer needed.
Now, it is perfectly OK to have two (or more) root partitions, each containing its own /boot directory containing the kernels for a particular Fedora version. When the Master Boot Record for the main disk is reconstructed to redirect to the newest /boot, it will also build a grub.cfg file that references ALL the kernels it finds, both new and old. That is, the new grub menu will contain menu entries for ALL the kernels that were found, not only the system just installed. This works for both old-style MBR partitioned disks and for newer GPT partitioned disks, which are required for UEFI systems.
Therefore, simply follow the standard installation procedure, selecting all relevant disk partitions for appropriate mount points, and allow the MBR to be rebuilt, pointing to the new root partition.
My preferred configuration is to have two separate root partitions and one /home partition. When a new Fedora release becomes available, I reformat the older of the existing root partitions and install the new system there. When the new installation is completed, the remaining older system must be adjusted to remove references in /etc/fstab to the deleted system before attempting to boot it.
With this setup, each system version remains entirely isolated from the other, with its own /boot directory and all the package management files seff-contained. Therefore, updates via dnf, even of the kernel, will affect only the root partition of the system currently running. It is perfectly possible to keep both systems up-to-date by booting one or the other and performing a dnf update.