Look at + & - voltage as simply "frames of reference".
Where you have 24volt potential difference measureed between the -12 and +12 terminals.
Where you have 12 volt potential difference measeured between Ground or Common point and +12
Where you also have 12 volt potential difference between -12 and Ground/Common.
The above readings assume you hook the red (+) lead of your VOM to the + side of the voltage and the black (-) lead to the more negative side terminal in.
Another way to look at a +/-12v power supply is in terms of a pair of 12 volt car batteries hooked up in series. The series voltage is 24 volts. The junction of the + side of the one battery to the - of the other is the COMMON point.
If you then put the negative terminal of the VOM on the COMMON terminal, you'll see +12v when you touch the RED lead to the unconnected positive terminal and -12v when you touch the RED lead to the unconnected negative terminal.
The fact remains that you have a 12 volt potential difference between the terminals of the battery. The + or - simply tells you which side terminal is at a higher potential difference of one side with respect to the other.
This same analogy works if you do +/-15vdc or +/-5vdc or whatever bipolar voltage that is needed or being supplied by the cells/batteries/PS.