Unfortunately it's been a while since I had to do a clean install of ubuntu, so I'm 'flying in the dark' a little.
however, these ideas may help you out....
You could probably start by checking the master / slave status of the disks that you are attempting to install to, it may prevent the second disk from being recognised during the install procedure?
If you are using the live CD you could check to make sure that the second disk is being correctly mounted (and visible) prior to instalation, you could use then check to determine the details of the second disk, using.
Another possibility, The system will want to write GRUB to the mbr on the primary hdd (or something similar... ?? ). Could it be that this is the stage you are at and hence you will only see the single drive?
Normally the install would locate your XP instalation and offer to 'shrink' for you and make space for the root partition.
You should then be offered to find all the disks and be asked where to mount what parts of the system, and how to format the disks (ext2 or ext3, reiser etc).
If your disk aren't being found at this stage you have a problem.
Check here for ubuntu community instalation instructions.
Otherwise if you are using a 'more advanced' install check this page
for the other options.
If you can clarify what instalatin procedure you are using, and what stage you are getting to (I'm assuming you are getting to the partition management stage) it will help to determine where the problem is.