The 2 stands for on 2 different mediums. So HDD and tape for instance. Or HDD and SSD. Or SSD and DVDs. Whatever combo you choose that fits your needs. This (minimizes) the chance of loss of both.
I’d love to use tape but so far couldn’t bring myself to make the Jump cause of the upfront cost of the drive. Other than that it would sound great to have tapes of my digitized bluray collection so as if my nas should fail unrecoverably, I could simply setup a new one and copy back the data instead of having to digitize everything again.