You can use microflow validation as described in https://docs.mendix.com/howto/data-models/setting-up-data-validation#5-advanced-validation-with-a-custom-save-button and have it trigger the popup message or validation feedback below input field when (either or) both input fields are empty.
hello Erupa,
yes, you can by creating a validation microflow that will check your expression if it's empty or it doesn't equal a specific value!
check the advance validation in this link: https://docs.mendix.com/howto/data-models/setting-up-data-validation
Best regards,
Mohammed Qadri