The same way you would add 2 lines to a plot that is not create using subplot - by either using hold or the following plot syntax:
x=-3:.1:3;
y1 = sin(x*pi/3);
y2 = cos(x*pi/3);
% option 1
plot(x,y1)
hold on
plot(x,y2)
hold off
xlim([-4,4])
% option 2
figure
plot(x,y1,x,y2)
xlim([-4,4])
