With alternating current, this is almost impossible without special equipment. In order to 'double the voltage', not only would the supplies need to be connected in series, but the phases would need to be syncronised.
Lets say AC power looks like this:
Not only does the current swing positive, but it alternates negative. (This property is in part responsible for the greater transmission distance of AC vs. DC power.)
In the image above, "Sine Wave 1" and "Sine Wave 2" are your separate power supplies. While they both output 12VAC, they are 180 degrees out of phase with each other. (Power Supply 1 is 'positive' when Power Supply 2 is 'negative') In this very simple model, the net transmission is 0 volts. (+12 - 12 = 0 = lots of heat instead)
In reality, the 'wall wart' power supplies won't be a nice 180 degrees out of Phase. It'll be something less, or even something that changes over time. So instead of nice, consistent, repeatable 60 Hz 24 VAC, you'll end up with some odd composite waveform that has two peaks of 19 VAC over a longer cycle of like 15 Hz. Not only this, but the efficiency will be horrendous, and you'll cancel out part of your desired output power no matter how efficient your supplies claim to be.
End result = a very unhappy electronic device. You can easily break the incoming voltage regulator on the camera even with ideal power. That thing gets obliterated with out of phase power within a few minutes.