I don't know the proper fix for that but I just wanted to give you a suggestion :
Boot into your Slackware install and install Lilo on your /boot partition NOT on your MBR.
Then when still in Slackware, open your grub.cfg file and create an manual entry that will make Grub2 (installed on the MBR) do a chainload to the partition where you just installed Lilo.
That way, you will let Lilo handle the boot process of your Slackware setup and use Grub2 for Fedora only.
Also, it will reduce the possibility of breakages
The addition of Grub2 to the main repository of Slackware is quite recent (mostly because of the UEFI-based machines).
That's one of the reason that Grub2 is not that well documented for Slackware.
Hope this helps