Two graphs sharing the same x axis

I have created two graphs which have number of observations as the x axis. I have combined these two graphs using graph combine and have one above the other. Is it possible to force them to share the same x axis (number of observations) but retain their independent y axes?

Any suggestions welcome.

Many thanks.
xcommon just ensures that they are both the same scale. What I want is just one axis for both graphs, then the y axis to be above and below the x axis, if that makes any sense at all!