No. The grounds are common between the supplies, and you would cause a short on the lower supply. You cannot "float" or disconnect the ground, as that would make it unsafe.
Each 12V output is connected to ground (not isolated)
The 12V output of a standard PC power supply cannot supply as much as 15A
To make a PC power supply work properly it needs a reasonable load (quite a lot of amps) on the 5V or 3.3V outputs as well.
