drupal_render does not render concatenated elements I am new to Drupal and wonder if users who have experience in this kind of questions can help me with my issue. 

The problem is that the output of $complete0 through drupal_render() results in an empty space. Replacing $complete0 with either $title0 or $name0 works just fine, however trying to render a concatenated field does not get the job done. Please see the piece of code below:

    try{
        $title0 = field_view_field('profile2', $user['recommendation'], 'field_rec0_title');
        $name0 = field_view_field('profile2', $user['recommendation'], 'field_rec0_title');
        $complete0 = "{$title0} {$name0}";
    }
    catch (EntityMalformedException $e) {}   
    if(empty($complete0)){
        $form['profile_evaluation']['field_field_eval_rec0']['und']['#title'] = 'Information missing';
    }
    else{
        $complete0['#title'] = 'Recommender 1';
        $form['profile_evaluation']['field_field_eval_rec0']['und']['#title'] = drupal_render($complete0);
    }
drupal_render() renders an array.

The var $complete0 in your code is a string, so you shouldn't be passing it to drupal_render().

Either use render(), which works for strings too, or convert $complete0 to a render array.