1250w ATX Power Supply Serial 24v Question
The only way way you could achieve a 24V output with PC or SERVER power supplies is to get one that specifically outputs 24V. 

You cannot even try to use two separate but identical PC power supplies that output 12V because the negative terminal of the 12V is always connected  to the common earth ground terminal at the input AC connection on the power supply. This means that you cannot stack the two 12V outputs.

So purchase the properly designed 24V power supply for you use.