It can be very dangerous. When you charge from a normal 240 volt outlet, the circuit is supplied from a ganged 2-pole breaker and if there is an overload or other problem, the breaker will trip and open both hot legs. When you use the 120 volt combiner, you are picking up two legs 180 degrees out of phase to make up 240 volts leg to leg, but each branch leg is on a separate breaker. Also, you don't know what other 120 volt loads are on each branch circuit and could end up with an unbalanced situation. In the event of overload or other trouble (caused by car charging or some other plugged in appliance, only one of the two separate 120 volt breakers will trip, leaving the other leg "hot".