The main reason people use two drives - a SSD and a regular HDD, is cost. The SSD is much faster as compared to the HDD, but the price is around $1 per GB for SSD, and $0.05-$0.10 per GB for HDD. So storage space is 10X - 20X more expensive for SSD.
My recommendation is to put the operating system, program files, and heavily utilized files on the SSD, and use a HDD for large files and files that aren't used often.
And always have a backup plan for both drives. Even the most reliable drives on the planet will fail at some point, it could be an immediate failure, or 10+ years - you never know. Without a backup, you lose your data.