Two 12 V heater elements in series in a dual hotend
No you don't want to do that. 

A 12&nbsp;V 30&nbsp;W heater has a resistance of about 5&nbsp;&ohm; (2.5&nbsp;A on 12&nbsp;V). A 24&nbsp;V 30&nbsp;W heater is about 19&nbsp;&ohm; (1.25&nbsp;A on 24&nbsp;V). Placing two 12&nbsp;V heaters in series means about 10&nbsp;&ohm;, for 24&nbsp;V that means that the current is 2.5&nbsp;A, similar to a 12&nbsp;V circuit, the power will be 30&nbsp;W for each heater. So it appears that this should work.

But, the problem is that being in series, both the hotends are heated. This is not beneficial for the unused core which is prone to ooze filament and can cook filament if not used for a long time (long stand-by high temperature). Typically, unused printing cores go to a lower stand-by temperature when they are not printing. Also it would be more difficult to have filaments of different temperatures in the hotends. Furthermore, which thermistor would you use? A hotend cools down by melting filament, the temperature drop is measured by the thermistor results in the control logic adding current to the heater to compensate the loss in temperature. If you only use one thermistor (basically, from a firmware configuration perspective, the setup is similar to having a single heater in a single hotend and having the filament being changed) and using the other core (without a thermistor) to extract filament, the temperature drop will not be registered and as such not controlled. There is no default firmware solution to use 2 thermistors in a "single" heating element (in this case strand of heating elements), this will probably require some modding to the source code of the firmware.

You could test this setup, but I would not use it for a long time.