chance for drive1 failing + chance for drive 2 failing so it is added.
(and multiplied for both drives failing if you like statistics)
Anyway a ssd is helping quite much in everyday stuff, and a normal hdd /nas for backup.
I run 2 sd and 2 hdd in my server, one of the hdd is already down after 3 years, the ssds are both fine, even. And I abuse them for ~ 10gb used swap space already, so pretty much full writes.
Btw a hdd is also limited, not in writes but in hours, as the mechanic stuff is wearing out till the disk some day cannot spin up anymore.