Hi eliasen, I'm not sure if this would fit your requirement; however to control in one place instead of two places, I would set as follows:
1) Assuming we have FieldA and FieldB. set Data Validation on FieldB by
2) adding validation "FieldB needs input when FieldA is not blank" with condition
FieldB is blank and
FieldA is not blank
3) and add one more validation "FieldB should be blank when FieldA is blank" with condition
FieldB is not blank and
FieldA is blank
Then, we get validation error for FieldB with red * mark (for FieldA is non-blank and FieldB is blank), or red dotted link border (for FieldA is blank and FieldB is non-blank), and data validation is controlled in one place.
FieldA does not get validation error with this style, but I think users can fix the issue with given validation message. This does not work if users really need error message both on FieldA and FieldB, but just as a potential way you may consider....
Another style would be : hide submit button if the condition is not satisfied. also condition should be clearly written in the form, but it seems still tricky...
thank you!