I have two internal SSDs in both my PC and in my Laptop.
One SSD is used for OS and files as normal. The other is used for automatic versioned backups of user files (not the OS) on the first SSD, every time I boot.
I also have two multibay USB DAS. Mergerfs and snapraid. One DAS is used mainly for media file storage and for backups of important folders on my PC and Laptop, as well as backups of my phone (mostly photos) and tablet. The other DAS is used mainly for backups of the other DAS.