Connecting two atx power supplies in series
As you can guess, you cannot connect two separate supplies in series if at least one point of them is common.

You know, ATX PSUs' outputs are isolated from mains:







simulate this circuit &ndash; Schematic created using CircuitLab

But you don't know that whether the output return path (i.e. GND) of any ATX PSU is connected to earth (e.g. via a series capacitor) or not. If return path is connected to EARTH, you cannot connect these two supplies in series. To avoid the risk of a failure, you should cut EARTH connection. But note that EARTH connection is of vital importance.