How can I set up two fields so one or the other must be completed

I've got a form almost finished but the last piece is two text fields that a user enters a number in one or the other.  How can I set these up so that it's required they do one or the other but not both and not leave them both blank, or can I?  This is not a webform but a template to email out, however I presume if it can be done they'd be done the same way.  Thanks