In this scenario you will need
three HDs.
One for Time Machine
One for the Library
One for the Library Back up.
FOr backing up the Library I would use an app like
DejaVu
FWIW: My back up routine:
My Library lives on my iMac. It’s Backed up to two external hard disks every day.
These disks are permanently attached to the iMac. These back ups run automatically. One is done by Time Machine, one is a bootable back up done by
SuperDuper
It’s also backed up to a portable hard disk when ever new photos are added. This hard disk lives in my car. For security, this disk is password protected. For this job I use
DejaVu because it makes a simple back up that is clear and can be tested easily without doing an full restore.
I have a second off-site back up at a relative’s house across town. That’s updated every 3 or 4 months.
My Photos are backed up online. Personally I use SmugMug but there are many options including flickr. However, check the terms of your account carefully. While most sites have free uploading, you will often find that these uploads are limited in terms of the file size, or the bandwidth you can use per month. For access that allows you to upload full size pics with no restrictions you will need to pay.
Every couple of months I test the back ups to make sure they are working correctly. It’s very easy to mis-configure a back up application, and the only way to protect against that is to do a restore.
Regards
TD