It will work if you get the phasing right. Something similar is done all the time in the power industry when small increases or decreases in AC voltage are needed (using boost and buck transformers). As transformer voltages can run a little high, you might want to be sure that the total voltage provided by your two transformers combined doesn't exceed the limits of the equipment you are powering.
As an aside, I have found thrift stores (such as Salvation Army, Goodwill) to be a good place to find plug-in transformers/power supplies. They usually have a barrel of them you can sort through and can often be bought for less than a dollar.
IPVMU Certified | 11/27/13 02:19am
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.
I know this isn't the answer you were looking for, but 24VAC plug in transformers are not that expensive at all, even brand new. $8 I think is what I paid last time I bought one. Seems a heck of a lot easier to do that.
Regardless of the theoretical possibility, we must consider electrical code concerns.
Off the top of my head, those Wall Warts are probably labelled as Class 2 device (see nameplate/pic below). Class 2 devices have voltage/current (VA) limitations they need stay within so that you can use there more lax wiring methods. (i.e. avoiding conduit/ac cabling, electrician qualifications, etc.).
In additon, the code likely has some "intended use" statements that should be considered.
I would suggest checking your local Electrical Codes first. See section 16 of Canadian Electrical Code. Maybe someone can help with NEC (NFPA 70)