It is not a matter of kiruv but a dispute as to whether the blessing(s) are to be recited as a unit (on the head and the hand) or if each of the two merits or requires its own blessing. Indeed, I have seen references in which both blessings are recited together. However, I do not have a link to that.
One explanation of this dispute is because:
Tefillin: A Primer
The two boxes represent the two ways that we serve God in this world:
thought (the head) and action (the arm). When putting on the
arm-tefillin, we focus on devoting our strength to the Almighty. The
head-tefillin imbues us with the idea of subjugating our intellect for
the love of God.
Thus, we consider the unity of our intellect and action, but they are two separate aspects of our life. We put the arm tefillin on first to show that we have said נעשה ונשמע.
We place the arm-tefillin on first, teaching that commitment to action
and implementation must precede thought. Why? Because Torah is not an
intellectual adventure of ivory-tower theoretics. Rather, Torah study
demands that we use that knowledge for the purpose of fulfilling God's
will in the world.
Brachot of Tefillin
Before the strap of the arm Tefillin is tightened, the blessing of
להניח תפילין is said.  If someone forgot to recite the
blessing before tightening the arm Tefillin one should still recite
the bracha, even if one has already placed one's head Tefillin.
Upon placement of the head Tefillin, Sephardic and Hasidic authorities
are of the opinion that the blessing on laying the head Tefillin is
not necessary and the one blessing on laying the arm Tefillin is
sufficient.  Ashkenazim do recite a second blessing of
על מצות תפילין on the head Tefillin, before tightening it around the
head.  It is also customary to add the phrase ברוך שם כבוד מלכותו
לעולם ועד because of the doubt as to the necessity of the bracha.
Shulchan Aruch 25:8
Kaf Hachayim 25:49
Shulchan Aruch 25:5
Rama 25:5, Kitzur Shulchan Aruch of Rav Rephael Baruch
Toledano, siman 10, laws of the donning of tefillin, seif 3. Kaf
Hachayim 25:40 writes that a Sephardi who hears the bracha on the
Tefillin shel rosh is permitted to answer amen in his head.
The reason for this difference is explained in Is a separate blessing recited on the head tefillin?
Although we put on two different tefillin, one for the hand the other
for the head, and in truth they constitute two distinct mitzvot,
nevertheless the blessing recited on the hand tefillin also cover the
head tefillin which are donned immediately afterwards. The sages also
instituted a special blessing for the head tefillin, which carries
more importance because of the different compartments and the extra
Shin (letter in the Aleph Bet) on the box.
There is, however, a difference of opinion whether this blessing is
added whenever tefillin are donned, or only if one mistakenly
interrupts between the donning of the hand tefillin and head tefillin
by speaking (which causes that the head tefillin is no longer
"covered" by the blessing recited before donning the hand tefillin).
Opinion 1: This special blessing is only recited on the head tefillin
in the event that there was an interruption between the donning of the
two tefillins. (And in such an instance, it is the only blessing
recited before donning the head tefillin.)
Opinion 2: The head tefillin always warrants its own special blessing.
If one interrupts between the hand and head tefillin, then both
tefillin blessings must be recited before donning the head tefillin.
[In all instances, after reciting the blessing on the head tefillin,
the following verse is added: Baruch sheim k'vod malchuto l'olam
va-ed. (Blessed is the name of the glory of His kingdom forever and
ever) This verse is normally said after mistakenly uttering G‑d's name
in vain, or saying an unnecessary (which contains G‑d's name). This is
now said in consideration of the possibility that the halachah follows
the first opinion, in which case a blessing was now recited "in
1. This is true whether one interrupted between the head
and hand tefillin or not. If there was no interruption, then the
status of the blessing recited over the head tefillin is in doubt. If
there was an interruption, then the necessity of the first blessing
recited on the head tefillin is disputed.