validate 2 fields in a form before saving?

[7_Ideas_Digital]

Hello everyone:
I have a problem with a "simple" validation in a form: 2 fields in a form, both are not required but at least 1 of them must be filled before saving the record
I'm sure this happened to more than one of you, seems I'm in a foggy day  today and I can't find a "simple" or logical solution.
Any ideas?

Thank you very much.