Back up options for a two disk synology NAS

Can you use a third NAS drive as a back up in a two bay drive such as DS224+ or 723+. Change them in and out and keep one off site rather than use an external hard drive?

Don’t do that, You won’t have a usable backup, and you will eventually break the NAS’ backplane.

Or backup to a second NAS or cloud service.

Buy a USB dock. Insert drive. Backup NAS using Hyper Backup.