Wire reqs for a 40 amp circuit
You should not even think of attempting to wire a 40A circuit with two 12 AWG (20A) parallel conductors. If you did this and then by chance one of the 12 AWG conductors failed to an open circuit for some reason you would have a serious overload condition possible on the other lone conductor. That conductor could very well overheat and cause a fire when the circuit is still backed up with a 40A breaker.