Seeing as Moshe commanded Yehoshua only much later - after Pasuk 28 in Va'eschanan (as recorded in Matos, Bamidbar, chapter 27), why is it mentioned here?
Oznayim la'Torah: Because of the principle 'Ein Mukdam u'Me'uchar ba'Torah' (and the Torah mentions it here in order to juxtapose Moshe's instructions to Yehoshua to his instructions to the two and and a half tribes).
Why does the Pasuk begin with "Sh'nei Malchei ha'Emori"(kings) and end with "Kol Maml'chos ha'Aretz" (kingdoms)?
Oznayim la'Torah: In keeping with Chazal, that the nations of Cana'an hired Sichon andy Og to guard them against their enemies because they were the mightiest of all the kings. Consequently the Torah is now telling Yisrael that just as they had defeated the two mighty kings, so too, would they defeat the other kings - who were so weak compared to Sichon and Og that it was as if they had no king. 1
See Oznayim la'Torah.
