It's generally a bad idea to have two system folders (same OS or different) under any MacOS unless one of them is OSX. I suspect that's the root of your problem, but it could be other things as well.
However, since you have each system on a different physical drive, which in theory is supposed to be OK, it may be something different.
Disconnect all perephrials except KB, mouse and monitor.
There is a keyboard shortcut, but only with "new world" powermacs (the option key); it won't work with your model.
You might try to open the System Folder and double-clicking on the System File to open it. If it opens sucessfully, you need to perform a clean install. If it won't open, drag it to the trash and then install a new OS over the old one.
You could try trasing the Startup Disk preferences (System Folder OS9: Preferences) and restarting to see if that helps. The natural result of that should be a blinking queston mark for a period until it finds a valid startup disk, then a normal startup. Try reselecting the startup disk then.
You could mount the CD (while booted from the HD), open Startup Disk control panel, select the CD as the startup disk, and then restart.
You could try pressing the C key on startup to force a CD startup.
In general, if there's a crash when starting up from CD, that points to something totally screwed up.
You could also try going inside the box and physically disconnecting the drive with 9.1 on it, which will force a start from the 8.6 System, assuming it's still blessed (ie it has the OS icon on the System Folder).
If the startup CD still crashes, try it the other way around (OS9 connected, OS8 disconnected). If neither of those work, you have something bigger than a software problem.
If they are SCSI drives, reversing the SCSI ID on both drives will cause the other one to boot first (it checks highest number first).
If it's not blessed, you need to drag the finder (System Folder OS8: Finder) to the desktop, leave it there momentarily, and then drag it back into the System Folder.
Are you getting a proper startup chime (chord, not individual notes)? A bad startup chime almost certainly points to a hardware problem (ie it failed the hardware check performed on each startup). It could be anything, but bad memory is one typical reason. Memory can deteriorate over time, or fail suddenly. It only takes one bad RAM bit to do it.
It's a longshot, but you might want to replace the PRAM battery, or simply reset the PRAM on startup (command-option-p-r).
You can also try to remove the PRAM battery, wait 10 minutes for the data to dissapate, and then reinsert it.
You could also reset the CUDA switch on the motherboard and try it then (a pushbutton on the motherboard, varies by model but you should be able to recognise it). If you replace or remove the battery, do it as well.