You should get/use one external drive for backups that you store separately (can be your 2nd or a new one). Having two separate internal drives for backup is not safe, as the system can damage data on both at the same time (e.g. malware/encryption, data corruption etc.).
RAID is for availability/uptime. I like to compare it to a shop system at the checkout. You can’t have shop payments halted if one drive fails, so you have a RAID. It allows you to repair/replace while the system keeps running and your business keeps operating. In a large business, every hour of downtime can cost you hundreds of thousand of currency, so RAID gets even more sophisticated. Downtime is not an option.
At home this is up to you. RAID can save you some hassle and grant performance, but likely costs you more money than it saves you. Backup is key, so have at least one separately stored copy and depending on the importance of your data, also have an off-site backup.