(Copied from comments section)
I'm wondering the same thing Jan asked. It should look something like this:
ax2 = axes(...,'Position',ax1.Position,...)
But that alone wont' align the two sets of x-ticks. You'll need to use xlim() to control the x axis limits and you'll need to set the xtick using set(h,'XTick', ...) to ensure there are the same number of ticks on each axis.