Edward - addaxis returns a handle to the axes, and not the line that was drawn on that axes. Since there is only one line drawn on that axes, h_2, we can use the Children property of the axes to get that handle to the line/curve. Try modifying your code as
h_2 = addaxis(x2,y2, 'b');
% get child line
h_2_child = get(h_2,'Children');
and then change the legend call to
legend([h_1' h_2_child], 'y1', 'ymin', 'ymax', 'y2')
Now, in the legend, the y2 curve will show with a blue line next to it.
