Rather than trying to extend a filesystem over two disks, why not just make a new, large enough partition on the second disk, then move your current /usr to the new partition. Reuse the vacated /usr partition for other purposes, or delete the vacated partition and enlarge the nearest partition forward of it to take up the free space.
After creating a new partition on the new disk and moving the contents of /usr to that partition, remember to edit your /etc/fstab to show the new device as the mount point for /usr.
Search thourgh the LQ forums. You should be able to find numerous discussions on the same topic, with instructions on how to do it.