Starting multiple distros from one /boot
 

I've reserved 10GB on my hdb to try out another distro (probably FC2). My current linux installation is also placed on hdb, but I've put a /boot partition on hda. Can I safely use this same partition to boot a second distro?

Reason I've put boot on the first hd is that I hid hdb from the BIOS to avoid windows drive letter changes because of second win installation on hdb and I wasn't sure if grub would load from a 'hidden' disk.

If I can't use the same partition (or maybe boot from hdb?) I'll have to make a new partition on hda (I left some space for this), but that would play havoc with my mount points, although I suppose editing fstab to reflect the changes will fix this (can anyone confirm?)

I'm no expert (at ALL) but theoretically you should be just fine, though I'd create another folder in, say, /mnt with the name of the new distro. If you install everything in /mnt/newdistro, i.e., as if it were /, you should be fine. You'd need to edit your lilo.conf to point at the kernel for the new distro and whatnot.

Although I'm not totally sure how to install a second partition on the same device as another the 'real' way; all I know is some distros let you choose your mount point during installation.

All though I'd wait for someone else to reply before you go doing this without backing stuff up.

Don't fancy the idea of installing to /mnt. I'd like to keep things as clean as possible and be able to delete any distro at any time without damaging the other.

Distro's I've seen so far (SuSE, Mandrake, RedHat) all let you choose mount points of you install in expert mode.

There may be a problem if two different distro's use the same name for the kernel or initrd files. Also, it is common for the generic vmlinz and initrd to be links to the actual files, so you probably will need to select the particular kernel instead when booting up. The second install may overwrite the links for the first. Sharing /tmp should be no problem, especially if it is cleared out after rebooting.
If you gave yourself a unique username each linux distro, you could use a single /home partition without any conflicts. Just don't format the /home partition when installing the second distro.

Many of the /sbin and /usr/sbin administrative commands like 'mkrescue' are actually scripts and may be distro dependant.

It would be possible to mount /usr/local on it's own partition. If you install programs there that you compile from source (tarballs), this partiiton may be ok to be shared. It is supposed to be save between upgrades. Often in a networked environment, the /usr directory is located shared a centralized location, while /usr/local is on the host, however, this is a case of a single distro on many computers accessing programs and maybe even the home directory in a central location. This makes it easier to install and maintain software and libraries in a single location once instead of on many machines.
Trying to maintain two distros on one machine may be more than twice the work maintaining software, and applying security updates. Although, I realize that you are just trying out another distro. I wouldn't recommend this as a permanant feature.

A seperate partition for /boot cures alot of headaches for me. I have two distros and bootable backups for both of them on a single hd. If the boot partition contains the initrd's and any other relevant files (bootsplashes etc) I have little trouble juggling the files. I have never used a distro specific kernel for any length of time, I have always built and patched my own, so AFAIK as long as the kernel is not too old it is ok to use on different distros.
Modules, since they are in /lib, have to be duplicated in each root partition to work properly. The distro binaries are built against the libraries in /lib and won't work without the specific build of the library it was compiled against, ie: A binary for slack will problebly not work in a gentoo environment. Sharing any partitions besides /home may cause problems.
To protect my kernel images I never mount /boot unless i need to. My /boot is actually a dos bootable 300mb win95 partition which can't be symlinked by nature. I still use lilo for convenyance, but if the lilo mbr was to fail, a guik fdisk /mbr to make it dos bootable again and then loadlin to fire up a kernel and I am back in business.
One thing I have not done is to use different versions of lilo in the seperate distros, I always write to the mbr from the distro with the newest lilo, even if the kernel is for the other distro. I also keep my lilo.conf on /boot with a symlink to it in /etc in all the root partitions so both distros could potentially have access to it if it were mounted.
Just for fun, any kernel can be redirected to try to boot to any partition by using the typeing root=<num> after the kernel label in the lilo boot screen at boot time. On my sys /dev/hdc2 is root=1602, hdc3 is 1603, hda1 is 0301. I don't fully understand the number designations, they maybe differnent for different ide bus implementations.
I have read many dual boot how-to's but I haven't yet come across a dual distro how to.
BTW you will be able to mount the second distro root partition in /mnt and chroot into it and use and configure it on the fly without rebooting. This enables you to use both at the same time in differnt virtual consoles.

Thnk you guys for your detailed explanations. It's gonna take a while to digest it all. I have to dive into GRUB (or Lilo) coniguration to learn how to include the different distros now. On the face of it I'd say I'll do a completely seperate installation, although might try to share /home. The fun is in the trying!