Which has a different failure mode to disks/NAS/fileservers.
AWS/Dropbox/Google/BackBlaze et al. are all at risk from credit card and/or financial failures. I can go broke and be unable to pay my bills, but my hard drives will still store my data.
Google and AWS (to a lesser extent) are also occasionally subject to arbitrary and capricious account revocation. Google specifically worries me there, there are way too many stories of people have problems with, say, they Google Play developer account or their YouTube account, and finding they're cut off from their gmail and google drive access, with pretty much zero way if fixing it unless you have a million twitter followers or a Google insider to go to bat for you.
3 Copies, 2 Media, 1 off site.
One should always have a local copy of their backups.
Lesson learned! I now still had several intact tape backups of my data, but no functional device that could read and restore from them.
>> The thing is, while keeping data on the same storage media, you may lose them due to the same hardware issues. In other words, you may lose two copies in the same accident. That’s why you should always combine media.
For "same media" the key part of this is storing the data on two DIFFERENT DEVICES of different types, they both could be hard drives, but they need to be in isolated systems of different types (say a Windows System and a Synology NAS, or a FreeNAS Storage Appliance and a Linux Server, etc)
The rule came about because people would use the same tape library, or have multiple copies on the same SAN (often times they would have a SAN Cluster of 2 or more devices that act as 1, and because they had 2 "devices" they felt they were protected.
Disparate media type (HDD, Tape, DVD, etc) may have some advantaged but as long as you are putting 2 copies on say your Home Desktop, and a NAS you have satisfied 2 media's even if both are using Hard drives
That said today the most common way for a person fill offsite and separate media is to use a Cloud Backup of some kind.