Require One Of Two Fields To Be Filled (either/or)?

Here is my issue that I am trying to resolve. I know how to make a field in a form required for validation however what I am trying to do is require that at least one of two available fields is filled in. The form performs a search using the 'first name', 'last name' or both. What I need is for the form to require at a minimum that at least one of the two fields is filled in prior to sending it to the script to do the search.Here is what it should do in each instance:

[first name] [NA] = form submitted
[NA] [last name] = form submitted
[first name] [last name] = form submitted
[na] [na] = Not submitted