Two nominations is indeed better than one. Unless you are holding an LOA conditional only upon receiving a nomination. Two gives the SA more flexibility in awarding you an appointment.
You would not "grade" better, no influence there. But having two or more is yet another slate of nominations from which you might be selected to receive the appointment.
In our state, the Senators sent a letter and said if one Senator gave you a nomination please call them to cancel your appointment with the other senator. The same with our congerssman. We were told you only need one nomination and a second would not make a difference.