One way to do this is by manipulating the XData and YData properties. Try:
figure; h1 = plot(1:10);
figure; h2 = plot(10:10:50,20:24);
set(h1,'XData',[get(h1,'XData') get(h2,'XData')], 'YData', [get(h1,'YData') get(h2,'YData')])
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