When root volume is in LVM over two encrypted partitions, only one is decrypted at startup

I installed (K)Ubuntu 8.10 via the Alternate disk and manually configured partitions so that there would be two encrypted partitions on two hard drives. The encrypted space on both was then set as volume for LVM, and LVM then made one volume group out of the two physical partitions. The root partition was then created in the logical group, so to access it both physical partitions must be decrypted.
However, only one of the two partitions has a prompt for the passphrase for decryption at boot, and the other is not decrypted. Boot then attempts to mount the root filesystem, which is obviously impossible as the volume group doesn't yet exist.
Eventually I am dropped into a busybox shell from where I can manually decrypt the second partition using cryptsetup luksOpen /dev/sdb2 sdb2_crypt and enter the passphrase. The volume group then exists, and if I exit the busybox shell the system boots as normal and works fine.

Essentially the boot up sequence needs to check if two or more partitions are required to make up the volume group that root is in, then ask for the decryption passphrase for all of them if required.