Well, if Mandrake isn't using GRUB already...
Boot Mandrake, and add entries for Red Hat in the /etc/lilo.conf file (assuming you're using lilo). You'll need to know where the Red Hat kernel is, and probably where the RH initrd is to (they start acting funny without their initrd's). Mount your RH partiton in Mandrake to look at it, and look at RH's /etc/grub.conf . You'll probably be able to gather the locations of the kernel and initrd from here. Add entries for RH like so:
Run 'lilo' to apply changes.
If it you're turns out Mandy's using GRUB, just cut/paste the entries from RH's /etc/grub.conf to Mandy's /etc/grub.conf