Salvando o objeto Mongoose em duas coleções

var RegistrationSchema = new Schema({
    name:{ type: String, default: '', trim: false}
});

var Registration = mongoose.model('Registration', RegistrationSchema);
var New_Registration = mongoose.model('New_Registration', RegistrationSchema);

var registration = new Registration(data);
registration.save();

var new_registration = new New_Registration(data);
new_registration.save();