Populate a field with the sum of two fields while content creation
Sounds liks a job for Computed Field


  Computed Field is a very powerful CCK field module that lets you add a
  custom "computed fields" to your content types. These computed fields
  are populated with values that you define via PHP code. You may draw
  on anything available to Drupal, including other fields, the current
  user, database tables, you name it. 


You should just be able to set the field value literally as in your example (i.e. field = fieldA + fieldB)