A couple additions for the above instructions.
If the knoppix installer gives you an option to do so, install Knoppix's boot loader to the partition that contains knoppix's /boot file. Not to the mbr. If you do this you won't have to reset Mint's bootloader to the mbr.
If you do have to rewrite the Mint bootloader to the mbr, do it from the Mint installation, not the knoppix installation.
I would suggest using the chainload method of booting knoppix from the Mint menu.lst. This way you won't have to modify the Mint menu.lst file if the knoppix kernel is ever updated.
Instead of putting the boot lines from knoppix in the Mint /boot/grub/menu.lst use the lines below. Put them after the line: (example root numbers)
### END DEBIAN AUTOMAGIC KERNELS LIST
title (these two lines are just a spacer line)
title Knoppix (whatever you want to show on the screen)
The root is the hard drive and the partition numbers that the Knoppix /boot file is in. Remember that drives and partitions start counting from zero, not one.
Sometimes the version of grub used by different distros is not compatible and it will be necessary to do the below commands from the Mint install in order to be able to boot the second distro.
Open terminal: (example root and setup numbers)
root is the drive and partition numbers that Knoppix menu.lst is in. setup is the drive and partition numbers that Knoppix /boot is in. Remember, drives and partition numbers start at zero, not one.
Insanity: Doing the same thing over and over and each time expecting a different result.
Democracy is 2 wolves and a lamb voting on the menu. Liberty is an armed lamb protesting the electoral outcome. A Republic negates the need for an armed protest.