If the hard disk has not been used yet, you will need to:
1. Use fdisk to create a new partition on the second hard disk
2. Format the new partition using mkfs
3. Create a directory where the new partition should be mounted, such as /home2
4. Mount the new partition in /home2 and set it up in fstab to mount at boot time.
5. Activate quota support on the new partition (if you want to enforce disk quotas)
6. Configure WHM to put new accounts on the new partition, this can be done in "Basic cPanel/WHM Setup" in WHM
As you stated, this will put new accounts on the new disk. To alleviate usage on the primary disk you could also move some large user accounts to /home2 and create symlinks so that their original home directory path of /home/username/ will point to the new location.