Shorting transformer leads
You definitely do not want to short the +12 and -12 outputs together. I don't see any reason to believe that using the center tap with one of the others will reduce available current.

If you need DC output from the transformer, you can use all three outputs to get full-wave output with (marginally) less circuitry than a full-wave bridge requires.







simulate this circuit &ndash; Schematic created using CircuitLab