I don't know grub that well, but I get along fairly well with Lilo. I would install Debian first, and then install Slackware. When you install Lilo, do an "expert" config, which will ask you to manually select your linux partitions. Simply select the Slackware partition first, and then add the Debian partition (assuming you want it to boot Slackware by default, otherwise reverse the order).
If for some reason that does not work, then when in you install Slackware just let Lilo automatically configure itself. If it doesn't pickup the Debian install, you can then manually edit lilo.conf and add it after the fact.
If you have trouble configuring Lilo, ask here.