How can I force one of two KNOPPIX filesystems to be detected

[szzsqing1]

If you look at /init file of Knoppix you will see the reason. Init file accomplishes this:

# Its purpose:
# 1. Mounting CD, DVD, Flashdisk or network drive,
# 2. unifying read-only compressed file system and ramdisk or file overlay,
# 3. give control to the real sysv init, which then continues to work like
# we booted from a single disk partition.

Init file is part of the initrd (in a running Knoppix OS it is: /init). You have to edit initrd to edit init file. The problem is that Knoppix's busybox (which is also part of initrd) does not support UUID detection. Instead, it uses the notations like /dev/sda, /dev/sdb ... But this method is not reliable: it can produce problems in case of multiple internal disks, external drives (USB stick) if more of them contains Knoppix OS because the first found instance will be used. The ideal solution would be that Knoppix use a busybox version that supports UUID detection so that can be applied a cheat code to give the UUID of boot device. Should be done a feature request...

But even in present situation you can solve this problem if you do not use ISO boot. Look at Knoppix cheatcodes.

### Configuration / Persistent image ###
knoppix knoppix_dir=KNOPPIX Directory to search for on the CD.

So, you have to rename your KNOPPIX directories with unique names and give the desired name as boot parameter. Of course, in case of Knoppix ISO this is impossible.
You can use this solution only if extract the ISO content and use your own boot parameters.
For example the ISO content is copied to a USB stick and you boot Knoppix with GRUB2 installed to that stick. And the boot params are given in grub.cfg file.