I have a dropdown menu like this:
I think that the require_if validation accept only one value per time. Try to change your validation code as below:
$rule = array(
'selection' => 'required',
'stext' => 'required_if:selection,2|required_if:selection,3',
);
EDIT: Check LePhleg answer, is more cleaner. At the time of the answer that was not possible, just check the question, he was using the same method but not worked.