This page is intended to assist with the upgrade of resources to infinite that service more than one city.
The mines that serve two cities are the fastest ones to run down (because you are depleting them twice as fast). Making them infinite saves you a huge amount of cost in the long run. Plus you double the effectiveness of your experts as two cities will get the benefit of an infinite mine.
In the current version of the game there are no dual location Sphene mines.