I would assume the debian installation uses a different kernel version. If you were to copy the RedHat entry and change the "root" partition, you would also have to put a copy of the Debian kernel in your "/boot" partition where grub resides on your RedHat side. You would also have to have a copy of the initrd image if your Debian uses one.
However, probably the easiest thing would have been to install Debian and had it install LILO to the partition boot sector on whichever partition your Debian is installed on. Then you would just need an entry similiar to a Windows Boot entry in GRUB's menu.lst file. All of this is speculation as I have not tried it but I think it should work. I personally switched back to LILO with the SuSe animated menus on my RedHat 7.2 boxes although GRUB seemed to work well.
[ April 29, 2002: Message edited by: VoidMain ]