Thanks for your input! I succesfully set up Slackware and Debian sharing the same kernel anyway. Very easy to do.
In short, this is how I did it: (just in case you're interested)
1. I repartitioned my /dev/hda:
Device Boot Start End Blocks Id System
/dev/hda4 1 2697 21663621 5 Extended
/dev/hda5 1 63 505984+ 82 Linux swap / Solaris
/dev/hda6 * 64 69 48163+ 83 Linux <--my bootpartition
/dev/hda7 70 945 7036438+ 83 Linux <--Debian
/dev/hda8 946 1821 7036438+ 83 Linux <--Slackware
2. Installed Debian on hda7 with hda6 mounted on /boot, and Grub on the MBR
3. Installed Slackware like I normally would on hda8 (No LILO install off course)
4. Moved hda8/boot to hda8/bootbackup and mounted hda6 to it in fstab
5. Edited Grub's menu.lst to boot from hda6
6. Moved hda7/lib/modules/2.4.27-2-386 to the bootdisk and created a link to it in both hda7/lib/modules and hda8/lib/modules.
(I put a detailed follow-through on my site