Hey OP, first off we want to thank you for your support with the drives you've currently got.
As far as backups, the most widely recommended backup strategy is called the 3-2-1 method:
Keep 3 copies of your data - 2 locally but on different mediums - with 1 stored offsite in case of disaster.
The thing you'll have to keep in mind with RAID, & forgive us because you will hear this often in forums, is that RAID is not a backup. RAID is designed to prevent system downtime when you simply can't afford the time/money that it being down means. Picture that you own/run an online business. The server that runs credit card transactions for this business simply can't afford to be down for a long period of time due to something like a hard drive failing, because this means money being burned in your pocket. That's where a situation for RAID comes in. It allows the system to keep running while you swap out a drive and rebuild the data structure around the new one so that the situation is relatively harmless in the bigger picture. The complication with backups this way is that, when you're talking about rebuilding multiple TB's of data bit by bit, errors can happen as part of the rebuild process which then lead to corrupted data. This is different than having separate, fully independently intact copies of the same data on different mediums or storage devices.
For that reason, it may be a good idea to come up with a solution that allows you to have copies of your drive in your internal computer system, that backs up to a large external drive, ideally one which you then store at another location for that last step in the method.
One thing you can do is sort the things you need throughout the drives, for example one drive has the video editing stuff, then you rename the partition as something like "Video stuff" for easier use, the other drive can contain video games (just an example, I don't know what you really use or do) etc, that way if you sort everything properly, you should be finding yourself in less situations where you're looking for something and just can't find it. That's one way to sort things throughout the partitions. For the backup, it's best that you get a large capacity external hard drive for backing up everything. I'm not the best when it comes to the first question though, so you should follow the instructions of the people who posted before me (and after me if there's going to be anyone).
