Positive-negative voltage from dual supply
Standard computer ATX power supplies are not isolated contrary to your understanding. The mains plug earth/ground is internally connected to black ground wire of the output side.

Therefore, if you connect two ATX computer power supplies to earthed mains sockets, the black ground wires are already referenced to same potential as they are connected together via the mains earth, so they cannot be connected in series to create +/- 12V supplies, or 24V supply.