If finding a power supply with two isolated 12V outputs proves difficult, in many cases it's fairly simple to add an additional output. The transformers in switchmode power supplies tend to have a high volts per turn ratio, so you only need a few turns of wire on the core to get 12V. Assuming there is room on the transformer, just add the necessary number of turns using magnet wire sized for the current needed. Add a fast rectifier and filter duplicating that of the existing 12V output and you should be good to go. In almost all of these, only one output is monitored for regulation, the rest of the voltages follow. There are a number of articles out on the web about modding surplus computer PSUs for other purposes that detail this sort of thing.