For a sefardi that only says one beracha on tefillin, can "Amen" and "Amen - yehe scheme raba..." be answered immediately after putting on the shel rosh but before tying the shel yad around the fingers? Or must he wait until the tefillin is on completely?
Since the shel rosh has been put on as well, is it considered that the beracha has been fully utilized or is there still the aspect of tying the strings around the finger that is needed to be completed before an interruption is allowed?