I think it's possible by extracting the data directly from plots; lets say you have two plots in different figures, get all the line objects, extract the x and y data from line properties and plot them in the same plot with hold on feature, here's a little example:
a= randn(1,100) ; b = randn(1,150) ;
h(1) = figure; plot(a); h(2) = figure; plot(b);
handleLine = findobj(h,'type','line');
hold on ;
for i = 1 : length(handleLine)
plot(get(handleLine(i),'XData'), get(handleLine(i),'YData')) ;
in the image I posted i've changed the marker and color of 1 line for clarity purpose.