Not sure what to say, but this is how it works on all the large systems I'm familiar with.
Image you have two servers, each with two 1TB disks (4TB physical storage). And you have two distinct services with 1TB datasets, and want some storage redundancy.
One option is to put each pair of discs in a RAID-1 configuration, and so each RAID instance holds one dataset. This protects against a disk failure, but not against server or network failures.
Your other option is to put one copy of the dataset on each server. Now you are protected from the failure of any single disk, server, or the network to that server.
In both cases you have 2TB of logical storage available.