I've installed Ubuntu 12.04 Server using the encrypted volume with LVM option. I've now added another harddrive, that I wish to encrypt using the same key - and only supply key once during boot.
How can I accomplish this?
This isn't exactly possible (with my limited understanding of these systems).
An alternative would be to add a keyfile on your first encrypted partition that will give access to the other drives.
When setting up a keyfile, make sure you investigate and figure out a good size, as well as measures to protect it from getting stolen. Also, I'd recommend adding a passphrase that you can remember in case the first drive fails.
The Archlinux community has a very good set of documentation in regards to setting up and managing LVM and cryptfs/dm-crypt/LUKS partitions.