Multiple widget for same field?
By using form_alter hook you can alter the type of field. I am assuming you are using Drupal 7. The below code will alter the field_working_as List (text) profile field in user_register_form in to checkboxes.

And the same field will show as selectbox in profile edit page. I hope this will help you.

function mycustommodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if($form_id == 'user_register_form') {
    unset($form['field_working_as']['und']['#options']['_none']);
    $form['field_working_as']['und']['#type']='checkboxes';
  }
}