ls -l /dev/disk/by-uuid will get you the uuid of the second hd partition you want i.e. "8492dc43-afab-4b5b-b169-b55f5b1cb87b -> ../../sdb2" for me.

edit /etc/cryptab add an extra line for the other parition, i.e. "luks-8492dc43-afab-4b5b-b169-b55f5b1cb87b UUID=8492dc43-afab-4b5b-b169-b55f5b1cb87b none" for me.

etc /etc/fstab add an extra line for the new mount partition, i.e. "/dev/mapper/vg_tedsx61s-lv_root /backup        ext3 defaults        0 0" in my case.

To create a new encrypted HD, you should be able to just follow the directions below in the posting about making an encrypted usb flash drive.