So you mean a Raid? (E.g. Raid1: 2x 1TB disk -> storage space of 1TB, but one disk can break without data loss).
Or combine two hard disks into one partition (e.g.: 2x 1TB disk -> partition of 2TB)
There are different solutions. Software or hardware (e.g. Raid controller, if you mean a Raid).
Windows offers the option "Storage spaces" in the control panel (display: small icons), this could possibly help you.