You should have back-ups of each of your Libraries and all of your Referenced Originals. Two full sets, with one set kept off-site, are recommended.
There are too many combinations and ways of backing up to provide specifics, but the general rule holds: back up your Library and back up all of Referenced Originals.
So if your Library is on your system drive, and you have Referenced Originals on MyExternalDrive1, then you need to have a backup of your system drive (Time Machine does this by default, when enabled), and a backup of the files on MyExternalDrive1.
From your description it sounds as though you have two backups of your Library (via Time Machine to a dedicated disk, and "backed up to another external drive that is only plugged in ... ", and no backup of your Referenced Originals.