I have this Debian machine I run headless. It is nominally a server although I don't do anything heavy with it. It is backup for my scanned film archive and I sometimes use it for long running image processing. (hours or days)

I messed up the package management by mixing stable with a more experimental repo. Now aptitude can't do anything because it can't resolve the dependencies. (This is not my question. I have the dependency issue posted elsewhere.) The best advice I have gotten on this is to start over.

This server has a number of custom configurations that are naturally poorly documented. It has a working VPN setup. It has working hibernate. There are other things as well.

So what I was thinking was to set up a dual boot. One with the existing server, and another with a fresh Debian install. As I configure the fresh install, I can refer to the old server. Also if I need something that isn't set up on the new server, I can boot the old one, do the task, then boot back.

Most dual boot tuts assume you want to dual with Windows or Mac, not two Linuxes.
My plan is to move the existing image to a bigger HD and then set up the dual. It is this latter step I don't know how to do. Can anyone point to a guide that talks about setting up a 2nd OS next to an existing Linux? Is it as simple as setting up the partitions and pointing the installer at the right place?

Yes, that is it. Only other consideration is where and what boot loader are you going to use. With one system installed, you have a boot loader. After you install the second system on another partition(s) add whats needed in the boot loader for the new system.

I have 4 Slackware systems in one box. Slack uses lilo on bios systems, so its easy to add a new one if I choose to. You can put a boot loader on each hard drive also, and if your BIOS allows you to select which drive to boot from, you can boot with any boot loader on any drive.

I suspect Debain uses grub. I have little experience with it, so you should look in the docs for information on how to add another boot entry.

If your system is UEFI, then there is doc online on how to multi-boot.

1 members found this post helpful.

I too have never used Debian, but for years on Ubuntu my modus operandi was to install the next version in a new partition and flop-flop between them in need. The update tool was pretty crappy and I used a common /home until I was satisfied all was ok and I could delete the old system partition(s).
Works similarly for Fedora, so "just do it". os-prober will sort out the boot menu for you.

1 members found this post helpful.

Sort of. It's how I would, though not necessary, as the Debian installer can do the repartitioning if you prefer. I recommend maintaing more control by doing it yourself in advance, then during Debian installation, selecting manual partitioning, which will consist mainly of selecting mount points for the partitions as you have made them. Os-prober should automatically find the other installation and include it in its boot menu.

You shouldn't often have to boot back and forth. The old / can be mounted on the new installation any place you find convenient, for live referral to its configs using sudo, su - or root login.

Note that I have no PCs with as few as two Linux installations. 12+ is average here.

1 members found this post helpful.

Moving distro is usually easy if you have a monitor, not sure if easy headless. Use a clone operation to move.

Making the second OS install is usually just install to new or clean media. Grub usually is correct.

If you have physical access I'd move to new drive, test it for working. Remove power/data to drive and then do clean install on new/original drive. Then use boot choice F key to select over dual boot. Not always 100% if you do updates on bios and uefi mixed distro's.