There are several ways to accomplish this, but they all boil down to defining a position for some text.
Probably the simplest way is that if you don't have a label for the x-axis, is to use
xlabel('This title is really an xlabel')
The title() function automatically places the text at the top, but you can adjust its position to place the text anywhere:
figure
plot(1:10)
title('Test')
set(get(gca,'title'),'Position',[5.5 0.4 1.00011])
Another way is to just place your "bottom title" with the text() command:
figure
plot(1:10)
title('Top title');
text(5, 0.4, 'Bottom title')
