The Kli Yakar asks how is it that Moshe could deviate from what Hashem commanded – isn’t there an issur of bal tosif? I don’t understand his question. The issur of bal tosif is to change a mitzvah. “V’samach yadcha” is nowhere counted as one of the 613 mitzvos, so what's the problem?
(R’ Shteinman in his Ayeles haShachar here answers the question by distinguishing between a command given to an individual and a command given to all of Klal Yisrael. It seems to me that his chiluk focusses on the symptom but not the root cause. Something told to the individual is by definition not a mitzvah –- that’s why there is no bal tosif. The size of the audience is just a siman, not the driving force.)
Secondly, adding to what Hashem says is not always bal tosif. If I eat more than a k’zayis of matzah on Pesach night, obviously there is no problem of bal tosif. The shiur is a minimum kiyum, not a maximum limit. So too here, Rashi’s borrowing of the term “b’ayin yafeh” from the world of commerce in Baba Basra or from the world of hafrashas terumah indicates that we are dealing with adding something above and beyond the minimum requirement, not passing a maximum threshold.
So much for the technical details; now let’s get to the crux of what’s going on: what difference does it really make whether Moshe put one hand on Yehoshua’s head or two?
The Shem m’Shmuel suggests that Moshe understood that the next generation, the generation that would conquer Eretz Yisrael, would need two types of leadership: 1) military/political/social/economic leadership to build and run the country; 2) spiritual leadership to continue the mesorah of Torah. (See the Derashos haRan on Parshas Shoftim.)
Hashem told Moshe “v’samach yadcha,” place one of your hands on Yehoshua – give him one of those two elements of leadership. Let him wage war, let him become the Prime Minister and figure out how to build the country. However, “lifnei Elazar hakohen ya’amod,” let him defer to Elazer in spiritual matters. Even without the daf yomi, the Shem m’Shmuel remembered Rashi in Eiruvin 63b that explains this pasuk to mean that Yehoshua would have to ask questions of halacha to Elazar. Elazar, not Yehoshua, would be the “ba’al hamesorah,” the leader responsible for the transmission of Torah to the next generation, and in this way something of Moshe’s legacy would remain for his family, the kohanim, who would be the primary teachers of Torah (see Rashi 27:23).
Moshe Rabeinu, however, decided to give both aspects of leadership to his talmid Yehoshu; he placed both his hands on his head. Not only was Yehoshua the great general who conquered Eretz Yisrael, but as we know from the first Mishna in Pirkei Avos, Yehoshua was the next link in the chain of mesorah of Torah as well.
This was not done for Yehoshua’s benefit alone, writes the Shem m’Shmuel, but also for the benefit of Klal Yisrael. By combining both roles in one individual, Moshe ensured that the political/economic/social development of Eretz Yisrael would be inseperable from and intertwined with Torah.
Need I spell out the lesson for our times where Torah leaders want no part in being generals and generals don’t want to hear about Torah? Where the seperation between Torah scholarship and service to Eretz Yisrael is seen as an ideal rather than a failure to have an integrated whole?