What would happen if this circuit was implemented practically with batteries? This circuit makes no logic to me because how can the same node be at two different electric potentials?. But i tried thinking about it and here's my 'naive' explanation: The 12V battery will start supplying a current of 12/1000 = 12mA, the 6V battery will supply 0 current, until the 12V battery starts to drain and lose its voltage, when the 12V battery reaches 6V it will still be supplying a current of 6mA, but the original 6V battery will start supplying a current of also 6mA, so we will have the same current that was supplied by the 12V (12mA) battery alone the moment the circuit was connected. If we used DC sources that maintained constant voltages (not batteries) then the 6V source would have no contribution since it won't supply any current. that's my naive explanation....so tell me what is your explanation?