Yeah OP you would want to have the 2 secondary output's wired more like this below, and pay close attention to the polarity of the windings, you want basically 2 batteries stacked on each other, so in essence 0V +12 +24V, or -12V 0V 12V once you call the center tap the common point.
Also pay very close attention to the diode and capacitor polarities, and remember, just considering the peak voltages, it's just 2 batteries stacked constructively together, and so no one should blow up their cap's, if the bottom of the transformer was called 0, and the middle was called +12V