Check the note below that section. It will attempt to unlock all root partitions with the same passphrase entered, so if you setup the second drive with encryption it will unlock alongside your primary drive. With LVM on top you should then be able to expand the VG to the second drive to add space or redundancy to your LVs.
If the second drive has subpartitions (LVM on bottom, encryption on top) it looks like you'll need additional setup (section 7.2.2) https://wiki.archlinux.org/title/Dm-crypt/Specialties#Modifying_the_encrypt_hook_for_multiple_partitions