Validating two inputs in CKEditor dialog

I've gotten the custom validator to work on each field, but they should be mututally exclusive: how to validate one field against the other?

URL: [input1] OR Keyword: [input2]

And I can validate either one with:

setup: function() {...},
validate: CKEditor.dialog.validate.functions(function(val) {
  var input1 = val; // value of THIS input field
  var input2 = ???; // value of the other input field?)
  return (input1 == "" || input2 == "");
},"Error message here"),
commit: function() {...}

Simplified example, but you can see what I need. A pointer to the other field from inside the validator.