Check this out
PartGUI. This partition tool works great if you really want to learn about partitioning in linux without using the disk-druid X windows or fdisk / parted. Alternatively, u can try out this
QTparted which what i'm using it right now. But beware! you probably have to allocated the other distro in first 1028 disk cylinder (limitation in bios MBR) to avoid problems and please retrieve the hardcore slackware to help you workaround with this matter.