Try getting rid of the ticks on one x-axis (the one that hasn't been dateticked):
set(ax(2),'XTick',[])
You may also want to ensure that the limits are the same:
xl = get(ax(1),'XLim');
set(ax(2),'XLim',xl)
(I'm not sure which is axis 1 and 2, so you may need to switch them.)