Who were Eldad and Meidad?
Targum Yonasan: They were the sons of Elitzafan ben Parnach, 1 to whom Yocheved, Moshe' mother, bore them, after Amram divorced her in Egypt, following the decree to kill all the newborn baby boys, 2 and he married her. 3
Why did they remain in the camp and not go with the other elders to the Ohel Mo'ed?
Rashi and Rashbam (citing Sanhedrin, 17a) and Targum Yonasan: They were originally chosen to become elders, but they remained in the camp, because they considered themselves unworthy of becoming communal leaders. 1
What did they prophecy?
Rashi (on Pasuk 28): They prophesied that Moshe would die and that Yehoshua, would take Yisrael into Eretz Yisrael.
Targum Yonasan: Eldad prophesied that Moshe would die and that Yehoshua, would take Yisrael into Eretz Yisrael and inherit them the land, Meidad, about the quails and their consequences, 1 and both of them, about Gog and Magog and about Techi'as ha'Meisim. 2
Why were there two elders too many
Rashi: Moshe initially chose seventy-two elders, six from each tribe, even though he required only seventy elders, 1 and he knew that no tribe would willingly forego one candidate.
Rashi: He therefore took seventy-two piece of paper, on seventy of which he wrote 'Zakein', and two of which he left blank, and after placing them in a box, he intended to ask each elder to withdraw one piece of paper ...
And what are the implications of "va'Tanach Aleihem ha'Ru'ach"?