PS.. One important consideration is that if connecting two power-supplies in series you have to 'float the ground' on the second power supply. If you dont know what this means then probably the safest thing is not to do it yourself because playing with HV electrics is dangerous.
In a nutshell, 'floating the ground' means isolating the negative DC output of the power supply from the ground connection of the AC power input. If you dont do that bad things will happen when you connect it up because the negative DC terminal of the second supply will be raised to 12V by the output of the first supply, which causes a dead short to the AC ground connection.