No, there is no line of code to achieve that. Merging of figures takes a bunch of work because of all of the special cases about menus and toolbars and callbacks (and because the two figures might have been created with different MATLAB releases with different support for graphics objects.)
In the simple case where you are willing to flub all of that:
first_fig = figure(1);
second_fig = figure(2);
first_ax = findobj(first_fig, 'type', 'axes');
second_ax = findobj(second_fig, 'type', 'axes')
if length(first_ax) ~= 1 || length(second_ax) ~= 1
error('this code requires the two figures to have exactly one axes each');
end
ch2 = get(second_ax, 'children'); %direct children only and don't try to find the hidden ones
copyobj(ch2, first_ax); %beam them over
