A second candidate has come forward for the leadership of the New Brunswick Green Party.     Roy MacMullin is the party's energy advocate and a former candidate in the riding of Moncton East.     MacMullin says he'll promote electoral cooperation among the Greens, NDP and possibly the People's Alliance.     The only other declared leadership candidate is David Coon, who has stepped down from his duties with the Conservation Council of New Brunswick.     The leadership convention is set for Sept. 22 in Fredericton.     The Greens have no seats in the provincial legislature.