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:
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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.
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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.