If you connect a load across the +12 and -12 rails you have a voltage potential of 24 volts. (Likewise, if you connect between, say, +12 and -5 the potential is 17 volts.)
The ground rail is just the PSU’s ground state, e.g. its lowest potential. So when you see wires labeled +12, -12, +5, -5 or whatever, that is always in reference to ground.
Thus, you will need to make sure all equipment in that circuit (PSU, motors, drivers, etc.) all share common ground. If two portions of your system have different ground potentials you’ll probably experience problems.