sure, just install the bootloader oc knoppix and the rest to their /.
then you just add the few lines needed to mandrakes bootloader. grub i think, this means you add them in mandrakes grub.conf.
nothing more to it. you could ofcource make a book about the details if you want. like sharing /home between them..
OH you need no separate Swap for each install, you most likely already have one for mandrake and that is enough, it wil be found and used.
just be certain to use the corect partitions for each install.
keep asking questions, i will try to answer.