mergeValidators.js

const FormAddon = () => {
  return(
    <div>
    <Field name = 'addonField' component={renderField} label="Addon label" type="text" placeholder="add on placeholder" />
    </div>
  )
}

const validateFormAddon = (values) => {
  console.log("validating!")
  const errors = {}
  if (!values.addonField) {
    errors.addonField = 'Required'
  }
  return errors
}

const validateMain = (values) => {
  let errors = {}
  if (!values.lastName) {
    errors.lastName = 'required...'
  }
  return errors
}

const mergeValidators = (...validators) => {
  return (values) => {
    let errors = {};
    validators.forEach( (validator) => {
      errors = Object.assign(errors, validator(values));
    });
    return errors;
  };
}

export default reduxForm({
  form: 'simple',  // a unique identifier for this form
  validate: mergeValidators(validateFormAddon, validateMain)
})(SimpleForm)