Connecting power supplies in series
Most likely the 12V power supply returns are connected to earth ground, and by putting them in series you're essentially shorting one of the rails out. You can only connect power supply rails in series if one of the outputs is 'floating' with respect to the other (no galvanic connection between them).

Most likely the 5V rail is floating with respect to the 12V, which allows them to be put in series.

You may want to investigate a boost converter, to bring the +12V up to whatever voltage you need. A single power supply may have enough capacity for your target.