Validate a group of fields so that form can only be submitted if 0 or 2+ fields have been completed
First I assume this must be for Drupal 6 since you are describing CCK fields? I would suggest creating a small custom module, and implementing hook_validate. 

You can then check how many fields have been entered by your user to see if it passes your requirements described above. 

If the validation fails you can then display the error to the logged in user with form_set_error

EDIT:

Since you're using Drupal 7, you can use hook_node_validate within a custom module.