We need to use 'hold on' for both the figure window. One example below.........
S=[1;2;3;4;5;6;7;9;8;10];
H=[1;5;3;4;6;6;4;2;2;3];
T=[1:10];
figure(1)
hold on
plot(T,S)
figure(2)
hold on
plot(T,H)
Now for next plot on both windows with holding we can plot....
figure(1)
plot(T,H,'r:')
figure(2)
plot(T,S,'k:')