I have exactly that, and grub has no problems with either, one of my disks is called sda and the other is sdb
i have sda1 sda2 sda3 and sdb1 sdb2 and sdb3.
boot is on sda1, i have gentoo's root / on sda3 and slackwares root / on sdb3. grub says
kernel (hd0,0)/gentookernel root=/dev/sda3 ...
kernel (hd0,0)/slackwarekernel root=/dev/sdb3
get the idea ?
once you have one linux OS set up you can mess about and create LOTS of different installations on as many partitions as you have. say partition your second or first disk with 3 primary partitions say sda1 2 and 3 then an extended partition with say another 3 sda4 - 6 and install a distro on EACH partition then put all the kernels in the one boot partition /boot or sda1 and just alter the /boot/grub/menu.lst file like above
kernel (hd0,0)/kernelname root=/dev/sdb6 or sdb7 or whatever
i have had several at once this way. When your installing another distro AFTER your main one, just dont install a boot loader, install the kernels and everything else and then just alter your current boot loader i.e menu.lst file to boot the new OS.
As an example lets say you partition your first disk with sda1 sda2 sda3.
You then install Fedora using these partitions. sda1 is /boot sda2 is swap and sda3 is your / root partition. All installs well and you decide to install say slackware
so you partition your second disk sdb into sdb1 and sdb2. You install slackware choosing sdb1 as your / root. and sda1 ( same as fedora) for your /boot and of course sda2 (that sda2 as in 'a' ) as your swap. Then all installs to sdb1 OK, and it installs the kernel in /boot (sda1), then when it asks , Do you want to install a bootloader / grub / lilo, you say NO!. Then alter the already installed menu.lst file ( from the previous fedora install say ) to say
kernel (hd0,0)/the-kernel-slackware-installed root=/dev/sdb1 <--- where you just installed the OS ( or more importantly... init ) and then reboot
Hope this helps, if not i can clarify anything you want me too.