Create a twin Axes sharing the yaxis.
Create a new Axes with an invisible y-axis and an independent x-axis positioned opposite to the original one (i.e. at top). The y-axis autoscale setting will be inherited from the original Axes. To ensure that the tick marks of both x-axes align, see
The newly created Axes instance
For those who are 'picking' artists while using twiny, pick events are only called for the artists in the top-most Axes.