You need regulators on the 12v and 6v outputs. Use a center tapped transformer that has secondary of about 15 vac rms. Use a full wave bridge rectifier. Take the feed to 12v regulator from filter cap (>25 wvdc rated) on output of full wave bridge. Take the feed to 6v regulator from filter cap (>15 wvdc rated) on center tap of transformer.
Amperage rating of transformer, size of filter caps, and regulator size and heat sinking will depend on power supply max current requirements.