Does +12V to -12V = 24V?
Yes, if you connect your load between the \$+12V\$ and \$-12V\$ terminals, you will have \$24V\$ across it. That load will draw a current which is sourced from the positive terminal and sinked (?) by the negative terminal. The power will be the current multiplied by the total supply voltage.

The negative terminal can be considered effect a virtual ground which you can use as a reference point for all of your calculations. To convert voltages referenced to the \$0V\$ to be relative to the virtual ground you simply add \$12V\$ on to them.



Furthermore, if there is also a \$0V\$ terminal as well, and your load is also connected to that in some way, you have to measure the power for each rail individually (as @mkeith mentions in the comments) as some current may be flowing into/from the \$0V\$ terminal. You would treat the two supplies as if they were not even connected to each other and each has a separate load.