There is a label on most (if not all) power supplies that tells you the maximum amps for each voltage (3.3, 5.0, 12v).
Most 400-500w PSU usually have a rating of 17 amps or more on the 12v line - which is enough to run 2 chargers without problems.
Just a few notes for those intending to build one:
Firstly, there are several links around to show you how to build one, with the main difference being whether or not to use a load device (resistor or light globe) on the 5v line.
Generally, the 5v line WILL need some load for the PSU to provide enough voltage on the 12v line - how much load really depends upon the quality and wattage of the PSU you have. As an example, I've seen instructions call for the use of a 10ohm resistor, yet my unit runs a 2ohm resistor to provide the necessary load. Some PSU's do not require ANY resistors to work correctly - so there is no real "right way" to do it.
Finally, remember that these PSU's are really not designed for us to open them up and start mucking around inside - so be CAREFUL!
It is possible to make one work properly without opening the case (though it looks untidy) - but if you are going to wire everything up internally then make sure the PSU has been unplugged for a long time - as there as several large capacitors inside that are still capable of giving you a nasty shock.
It's best to completely finish the unit before switching on for the first time.
EDIT:
I have seen a lot of threads on the net turn into nasty arguments over the safety issues with building these PSU's so do not attempt this if you are not 100% sure of what you're doing.