Try something like this;
[root@localhost ~]# grub
Probing devices to guess BIOS drives. This may take a long time.
GNU GRUB version 0.97 (640K lower / 3072K upper memory)
[ Minimal BASH-like line editing is supported. For the first word, TAB
lists possible command completions. Anywhere else TAB lists the possible
completions of a device/filename. ]
grub> find /boot/grub/stage2
find /boot/grub/stage2
(hd0,0)
(hd0,6)
(hd0,7)
(hd0,8)
(hd0,12)
(hd0,13)
(hd0,14)
(hd0,16)
(hd1,0)
(hd1,7)
(hd2,0)
(hd2,2)
(hd2,4)
(hd2,5)
(hd2,6)
(hd2,8)
(hd2,9)
(hd2,10)
(hd2,14)
(hd2,15)
grub> root (hd2,5)
root (hd2,5)
Filesystem type is ext2fs, partition type 0x83
grub> setup (hd2,5)
setup (hd2,5)
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/e2fs_stage1_5" exists... yes
Running "embed /boot/grub/e2fs_stage1_5 (hd2,5)"... failed (this is not fatal)
Running "embed /boot/grub/e2fs_stage1_5 (hd2,5)"... failed (this is not fatal)
Running "install /boot/grub/stage1 (hd2,5) /boot/grub/stage2 p /boot/grub/menu.lst "... succeeded
Done.
grub> quit
quit
[root@localhost ~]#
Use the actual partition designations for your installations, as revealed by the first grub find command. Do the root (<whatever>) and setup (<whatever>) for each of the two partitions you want grub installed to.