When two ore fields are overlapping each other, it's not possible to choose what a mining drill on top of both will produce. I don't understand how the mining drill chooses the ore to produce, but it alternates from one to the other by itself. I find this really annoying because it forces the player to put filter inserters and the likes for sorting the output ore.
![Image](https://img15.hostingpics.net/pics/642525FactorioMiningDrill.png)
In the image above, the mining drill first produced copper (bottom of the belt), and then suddenly changed its production to stone.
It would be really nive if it were possible to choose the ore being produced (in the same fashion we choose recipes for assembly machines).