24v from 12v

Are you talking AC or DC here?
If AC, you can get double the voltage on the output by combining the output secondary leads in series (not parallel). The transformer connections have polarity themselves so you have to wire them correctly or you will get close to 0 volts on the output. The effect is similar to a center tapped 12/24 VAC Transformer except the coils are physically separated. Of course, if this is AC, you can just simplify your life and use a 12/24 VAC step-up transformer.
If this is DC, then you would might be faced with a bipolar grounding issue. If you put two 12v DC supplies in series for 24 VDC then your common connection is + -. Might be a problem if one side of either 12 V. supply is already grounded. It might cause a regulation problem. It's probably not a good idea to tie the supplies together like this.
A far better and more common solution is to use a DC to DC convertor. They are readily available to convert 12 VDC to 24 VDC and you can get them in a range from 2 to 150 watts. Make sure your 12 V. supply can take the increased current and also your cables. Category 5 cable is limited to about 15 watts or so.
One source is they have a huge selection
Beachcomber

Thanks for your info Beachcomber, I'm going to look for a 12/24 VAC step-up transformer.