Can webform conditionals require filling one of two fields?
The Webform Validation module has what you are looking for (Require at least one of two fields).


  The following validation rules are currently included: 
  Numeric values (optionally specify min and/or max value)
  Minimum length Maximum length Minimum number of
  words Maximum number of words Equal values on
  multiple fields Unique values on multiple fields
  Specific value Require at least one of two fields
  Require at least one of several fields Minimum number of
  selections required Maximum number of selections allowed
  Exact number of selections required Plain text (disallow
  tags) Regular expression Must be empty (anti-spam:
  Hide with CSS) Words blacklist Must match a
  username