Ah I understand now. The Required box is what tells the DB it's a required field, so one solution could have the field marked as Required, but set it as Not Enabled using the example I sent. If they don't ever enter a value, you could set the default to some blank space or something. This isn't a great solution because you potentially have erroneous data in the DB for your records.
I looked into making a Validation Rule for the field (trying to make it look at the "other" filed's value), but it doesn't seem to know how to find those values.
Only options I can think of are to:
Define a custom macro that your Validation rule could look at. Still, this may not work if it never executes the validation if the field is empty.
Write some JS to validate the values (looking at the field values) and injecting that into the form layout. This is probably the route I would take if the Required option above didn't work.