Can you combine two 12v DC 600mA adapters to make one 12v DC 1200mA adapter by combining the wires together and plugging both 600mA adapters in?
This might work if the output voltages and impedances of both adapters were identical.  In practice, they won't be (one might put out 12.0 volts and the other 12.2 volts, for example).  In that case, the adapter with the higher voltage will try to drive current into the one with the lower voltage which will cause problems because most adapters are only designed to source current and not sink it. You could try putting small resistors in series with each adapter so that their voltage drop would help equalize the outputs.  However, this will decrease the voltage regulation and reduce the output voltage.  You would be much better off getting a single adapter that can supply the required current.